Always try to stay one step ahead of your enemy. If he hasn't rolled out any current tech, he either sucks or he's saving his resources to get the top-tier stuff. And if that happens and you're not ready for it, you're fucked.
Example: A guy is playing Wehrmacht and he's just using infantry after 10 minutes into the game. This probably means he's skipping Tier 2 and going straight to Tier 3, which has some decent armor. If that happens and you don't have any AT guns or sticky bombs, then he's going to roll over you.
Also, a general rule of the game is that you should never, ever hold in on a position and wait for the enemy; always keep advancing and taking resource points unless it's obvious that an attack is coming and you want to hold the line.