Lot of strat guides have been already written and it's impossible to write a new one without mentioning things already said there. Also, there are few things missing there and there are some new strat tactics which beat the old ones. I'll try to forget what writes in previous guides and not to sound too repetitive...

I can't tell you what exactly to do, because there are dozens possibilities in every moment, some are good, some are bad, and realizing which is which is part of a learning process that every player has to go through by himself, because even in games you are losing and not playing well you learn something. Only thing that's important is that you have to change something in every next game you play. Until you reach pro level every game should be a test game.
I will edit this page from time to time, by adding things I consider important, but I forgot about them when I was writing this for the first time.
SCRAP
Scrap values on maps are all wrong (fixed in
BZ Installer), here are the real numbers for all stock maps:
Paranoia 560 scrap 4 players max
Canyon Madness 560 scrap 4 players max
Blast Chamber 524 scrap 4 players max
Big Venus Map 500 scrap 2 players max
Ice Ice Baby 480 scrap 3 players max
The Compass 480 scrap 2 players max
Citadels 440 scrap 2 players max
Z Neo-Halofexx 420 scrap 4 players max
Battle By The Sea 420 scrap 3 players max
Theater o' Pain 400 scrap 4 players max
Corner Pocket 400 scrap 2 players max
Par 3 380 scrap 3 players max
Moon 360 scrap 3 players max
Z Snowed In 320 scrap 2 players max
Purgatory 300 scrap 2 players max
Before entering the game do the math, scrap divided with number of players, that will tell you how much at least you have to collect, remember, what you don't collect your opponent(s) will.
How much more (or less) scrap you get when destroying something comparing to its build price:
-2 nsdf walker, barracks
0 scavenger, nsdf minelayer, cca walker
+1 nsdf bomber, supply
+2 turret, scout (fighter), tank, light t., rocket t., s-power, gun tower, comm tower, cca minelayer
+3 apc, hangar
+4 nsdf factory, armory, tug, howitzer, silo
+5 cca bomber
+6 constructor, cca factory
---
+12 cca recycler
+20 nsdf recycler
-2, 0 and +1 should always be destroyed when seen, but before destroying all other units you should think first who will collect that scrap, because you don't want to help your enemy to become stronger. Of course, recyclers, upgraded units and opponents vehicles are always a target.
NAVBEACONS
Navs are very important, you have to surround your base with them, to be able to navigate with minelayers when you are not there, and you have to drop them on important points, to be able to see your opponent movements. Also have them between hangars and supplies to be able to tell your units to repair/reload with simple order "go to nav".
MINELAYERS
Crucial units in your defense, but useless if not set on right places, constantly reloaded, and set on right places again over and over. The best thing for doing that is with navs. Minelayers lay 3 sets of mines but you have to remind them to do so 3 times. If you have supply near where mines are laid down it is the best to lay 2 sets of mines and then tell miner to reload. While he's reloading first set of mines will explode but 2nd will still be active when miner gets back reloaded. If supply is too far, tell another miner to take first miner's place.
There's a little armory trick most helpful for miners but can be used with other units as well. Keep your armory undeployed, like that you can repair/reload your units with simple repair/reload order, you don't have to send them to hangars/supplies by yourself or by using navs and you are not in danger that your (moving) unit is far from hangar/supply, because then (in case when armory is deployed) repair/ammo will be send to them and when that repair/ammo arrives who knows where that unit will be.
Minelayers (and some other units) often get stuck on a mountains(you will see them going up-down-up-down). There are 2 tricks how to set them free, for both you have to be there:
1. when they are on the lowest point tell them to follow you and repeat that a few times, it doesn't work out of first try, you need 3-5 tries
2. tell stuck unit to pick you up, when it slides down, go back to your prior vehicle
HOWITZERS
- spread them
- always have 2 or 3 of them in a group, use F key to group them
- tell them to attack something, they fire on their own only on close targets (200m), when you command them their range is 600m (even more when on higher ground)
APC's
– they are useless if you don't have barracks too. Try building hangar next to barracks so they could repair after they get home.
- when attacked by them kill those little pilots with mortars (I strongly suggest regular mortar, not MDM)
- when you can't kill them all with mortars, Bombers and Miners are best defence against APC pilots
SCAVENGERS
- guard your full scavs, if you lose them, 6 scrap they are holding will disappear with them
- set them to scav only when there's no danger, until the threat is present tell them to follow something (hangar, power...)
- when they get hit they stop collecting and they go toward rec no matter if they are (half) full or empty, that's why you often have to remind them to scav. Use one of the F's for doing that fast
- always have at least 5 of them and when you lose one build another one instant
ATTACKINGAttacking is the creative part of Bz strategy. It's the hardest part, but also it's the part that separates good players from bad players. Everyone (even DMer's) can build a nice base and wait, but only a pro can efficiently attack base-sitting enemy.
There are 2 rules that need to be followed, that lead to the successful attack... never attack without fully prepared outpost nearby and never attack alone. Your goal is to destroy enemy's base and stop him from collecting scrap that was made in that process. In other words, you need to collect that scrap, or at least the bigger part.
What fully prepared outpost means? It means silo+few scavengers (following something, not collecting until it's safe), hangar+supply, power(s)+tower(s), minelayer(s) between you and enemy, barracks+apc(s), few howitzers and the rest of your available attacking force. If possible (if enough scrap), having spare tank, also armed with dual SP's like your main tank, waiting between hangar/supply, would be a great addition and it will save you precious time wasted on reloading/repairing your main tank.
Of course, on the map without enough scrap it won't be possible to build such a big outpost, at least not right away. In that case you should build just what you need the most, for example silo and supply. Your enemy won't let you camp outside his base and soon he will start shooting your stuff, which will give you additional scrap for your outpost.
If your enemy is hard-core base sitter then your outpost should be howitzers mainly. The thing is that you have to adapt to the situation and build what's needed at the given time.
When you lose a tank, build another tank immediately, when you lose a scav, build a scav etc. Even while DM-ing with your opponent you have to rebuild your force. That will sometimes cost you unwanted hit, but it will pay off in the end. Waiting for the fight to be over to build something will slow you down too much. In real-time-strategy time is very important, sometimes just a few missed seconds can change the final outcome.
Few times I was accused for cheating because of my fast (re)build. I was building tanks with factory, scouts with recycler, while my opponent was
sleeping. When he saw me with 4 tanks and 4 scouts in just a few minutes he said something like "no way you can have 8 units so quick". hehe. Like always in that situation, I offered strat lessons, one guy accepted them... he's a much better player today.
DEFENDINGI won't describe base sitting strategy, because there isn't one, at least not winning one. You can build huge base and wait for attacker, but in the end you will be crushed. In Bz attacker wins. Of course there are exceptions, when defender is a pro and attacker is an amateur.
When you attack you adapt to your enemy, same is when you defend. If attacker has 3 howies bombarding your base, you need 3 howies to bombard his howies. The thing is that if he gets his howies into position before you even started building yours it will be very hard to remove them when they start firing, especially if they are well protected. So, when building base (whether you preparing for an attack or for defense), you shouldn't build 10 towers and 0 howies. Those 10 towers can be taken down in no time with just 3 howies. You should build few of each.
I personally don't use all units when playing... I don't think that I ever built light tank or rocket tank, but that's just me. There are players whose whole strategy is based on rocket tanks upgraded with comets. Anyway, there are units that you must build, especially when defending - miners. How they are properly used I already explained in minelayer's section.
GUN TOWERSNSDF tower is good enough stock, CCA has to be upgraded with dual blast. Upgrading is expensive and that is the main reason why most people play with NSDF. Of course, CCA has some other advantages, but about that some other time...
How you build your tower(s) is very important. 1 tower with 1 power presents no threat to opponent's tank with dual SP's. But 2 towers very close to each other and 2 powers in background (as far as possible, and split) are not so easy to take down, especially if mines are around...
Some players (Sir Stratking, BUM...) use lame tactics of jumping with their tank into your recycler. When they do that, your units in attempt to destroy that tank inside your recycler, shoot at your own recycler instead. You can call your mobile units back, but gun towers will keep shooting as long as they see tank in the recycler. So it is not smart to have a lot of towers around rec, cause the more you have the sooner they will destroy your own rec. So 1 tower in reach of your rec is enough, and place it so it is looking at recycler's most open part. Like that it is harder (but not impossible) for rec divers to jump and stay in without being hit.