Patience...and practice.
I don't agree completely with what Lando says about attacking a neighbor early. Attacking just to attack, or doing so because you're looking to gain a specific center, the board be damned, is bad strategy.
Generally, when I make an aggressive move towards a neighbor early, it's to make them appear vulnerable, by getting their units out of position. My goal is to make them look like a better target for a mutual neighbor. If that neighbor can see the opportunity to gain, let's say 2 centers in the next two years against the player I moved against, but has no guarantee of gaining a center against me, then more often than not, he's going to go where the builds are. When it works, I'm in position to either follow through with the "pile-on" alliance, or move on the mutual neighbor if he get's out of position going for those two centers.