Wait. Are you actually asking for "generic tips" on how to beat people better than you?
I've got one. Get better. 
(This post is very light-hearted - no cruelty untended)
Avoiding losing positions
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b3 is an excellent move. Your knight doesn't have any immediate threats with the rook on b1. Don't be afraid of ghosts and phantoms! He doesn't have activity on the queenside after c4.
You focus way too much on the b-pawn... I'd say let the bastard die if he's gonna cause that much trouble on your part. You could even have ripped the a-pawn off with 12 Bxa7 and you'd end up even. You give up any thought of attack to protect this little pawn which allows your opponent to build up his forces at his leisure. Give your opponent something to worry about. In chess it is certainly true that the best defense is a good offense.
My verdict on 9 Nxc6 is that, while it doubles a pawn, this is probably the most trivial pawn to double, having done it a lot myself. It really doesn't hurt black that much and as you see, it opens up his queenside rook. It also cuts off a couple of strong squares for your knight. I would have played 9. Nf5 which keeps the focus on the kingside, makes him think about defense instead of offense.
20. Rxe8 seems pointless. You traded off your most active piece for his third-most active, doesn't seem that great of a trade. You seemed kinda determined to trade off pieces for the sake of trading them off, but you were often trading off some of your most active pieces for his less active ones, and that's why you ended up in a position where all of his pieces were actively bearing down on you.
In some of my recent games, especially in those against players a couple hundred rating points higher than me, I play pretty well as far as not dropping pieces goes. However, they always seem to find some kind of weakness, attack it multiple times, and end up exploiting it. It's usually something like an isolated pawn, a backwards pawn, pieces dependant on each other, or something similar. Any generic tips for how to avoid this?
Here's a recent example. I was white.