Winning AB is pretty simple ...
Control 3 nodes for the majority of the match, and you win.
A mistake many make is that they get 3 nodes, and defend them, then fall apart when they lose a node. While you have 3 nodes, and the other has 2, of course they are going to try to get a 3rd. No team is going to sit on 2 nodes and ride out the match!
Unfortunately, you can't just put 5 people at each node and guarantee yourself a victory. They key at this point is communication and reaction.
When you lose a node, and you will, you either have to get that node back quickly, or take a node they just left under-defended.
Now, you shouldn't all mount up and charge the node under siege, leaving the other 2 nodes undefended! Big mistake often made!
Here is where communication comes in! If the node under attack is being hit with 5 or less, it can be salvaged. If it is hit with more than 5, you should sacrifice that node and take one of theirs. Stealth characters should keep the other 2 nodes under surveillance for a quick counter strike.
This is actually MUCH more advantageous to you than trying to hold the 3rd node!
Why? Because when you are up 3 to 2, you are gaining resources at a 3 to 2 ratio. When you swap nodes, you are gaining resources at a 2 to 1 ratio for the minute it takes to gain control of the node after capping.
Now, in many matches, you will at some point find yourselves down 3 to 2 instead of up 3 to 2. Don't panic! Don't go crazy and wind up down 4 to 1. At a 4 to 1 ratio, even for a couple of minutes, the other team will get a huge lead.
The key here is to scout, identify the weakness, then strike. BUT, don't just hit it straight out! Keep 2 at each node you hold, send 3 as a feint to one node, and 30 seconds later send 8 to the node you want to take. This will prevent their entire team from showing up as you are trying to take the node you want, as they will already be responding to your feint. For the feint, use hardy characters that can take a beating to drag out the battle.
Now, the coup de grace is establishing a lead with the above strategies, then, if possible, taking a fourth node when they are vulnerable. 1 or 2 stealthers can often grab a fourth node as half the other team rides off to do battle! This is especially effective at the mine as many players hate to defend this node, as there is almost no visibility towards the rest of the battlefield.
All of this is sound advice, IF you can get your team to listen. Many times just having a level headed leader who can direct the action is more valuable than anything else. If your team is wasting time typing out insults to each other, it's kind of hard to type and fight at the same time. Each second spent arguing is 100 percent wasted time!
And of course, even if you play a good game, sometimes the other team is just better. You may be out-geared, the other team may have a better mix of classes, and may just be more coordinated and skillful than you.
When you find yourselves in this situation, "Just let them win!" is not a constructive strategy. Throughout your BG career, you will wind up playing with many of the same players over and over. Fostering a losing attitude is not constructive at all.
Above all, remember that it IS a game, and play to win, but also to enjoy!