These are a few things I do, since I was always getting killed by those that built in on me. First, dock block the waterways leading to enemy. That is build one or two docks right on the waterway to block passage.
Second, wall in choke holds that protects your town and essential resources for your civ. Dont try to control 30% of the board while in stone or tool age. Just control what you need to advance.
Third, build a scout, and uncover all the black. Alot of players will place a peon on the edge and wait till they have the wood to build shops, find him asap. Also, better players like to build between you and your ally, that way the are central to 2 enemies, check there first.
Fourth, notice the animal formations, if they moved such as gazzeles or aligators, you got someone walking around, find him and kill him before he can build.
Fifth, build radar houses by water ways or key entrances. If you know someone is in you, that's half the battle, then send someone to check things out. Key is to find him before he can get units that can hurt you.
Last tip, when you dock block w/ more than one dock, start one, build alittle then start the other, nothing is worse than having a scout kill your peon w/ only one partially built dock. And try to build from outside, so that you can build in him or her! Hope that helps.