Some people in any sport or game or event are pros. At 0AD, I'm not one. People who you can't faze and know the subject with total intuition are always going to be heard to beat. That said, not many people ARE like that. And unless you're playing the Nadals, Klitschkos, or other champions of the gaming world, there are some good ways to win against most other folk.
Now, how is it done? Essentially there are three factors in winning at 0 A.D. The first is obvious; strategy.
Strategically you should be able to respond well to your opponent. The major step is scouting. You HAVE to know what your opponent can throw at you in advance of them doing it, and the more advance warning the better. Get the lie of the land sussed early on, and remember that it's never, never, never a waste to throw a scout into an area where you don't think they'd be likely to be. There are a range of strategic approaches beyond this, but all of them are improved by information. Read about ancient warfare; what worked for them can often work in simulation form as well. Don't make your plans too rigid, as the final point; a plan never survives contact with the enemy perfectly formed. If this makes the plan useless, it's a good sign that it was in fact a very bad plan.
Factor two now, speed (or at least, stopping speedgamers).
This is the one I can teach you least; it's a classic of RTS gaming that the best players are that way mostly because they react and build up extremely fast. 0AD is being designed to be as resistant to speedgaming as possible, though; generally, defensive play can absorb speed very well. Garrisoned towers and fortresses can do an awful, awful lot of damage to advancing opponents, and if anything 0AD is defender rather than aggressor balanced at present. If you need to speedbump an attacking force, garrison watchtowers and have cavalry hit their siege kit - ignore the troops and super units until the siege kit is down. Alternatively, use guerilla tactics - hit resource gatherers with cheap, weak troops just to hamper their economy and give you time to react.
Final factor, and the least well used; psychology.
The trick to winning a slow-paced RTS like 0AD is not so much about controlling your troops. If you can control your opponent, you've won already. You should be talking to them constantly, discussing the battles you've had, telling them your current situation, and so on. It's an imprecise art, but the trick is to make sure you tell the truth enough and lie at the times you really need to. Those shouldn't always be climactic moments, because major moments are when your opponent is likely to be most on guard mentally. It's for when you need to break a stalemate, or when you need to hide the location of a base, or trick your opponent into looking in the wrong direction, that you're most in need of this. Generally, a better scouted player always holds a mental advantage; if you can kill their scouts so they don't know what's going on, or if they just don't scout, that gives you a huge knowledge advantage in discussions.
