What's important:
-Wedges, like everyone already said.
-Low center of gravity. The bigger and heavier your hammer, the wider and longer your bot has to be to keep it stable.
-Trapping capabilities. There are two parts to this. One, you need a broad V-shaped front to keep bots from slipping to the side. And two, you need some kind of overhead stopper to keep bots from climbing up the front of the bot and hitting the hammer arm.
-Good armor. Since the front of hammer bots is usually neither wedged nor protected by the weapon, it needs extra protection.
There are exceptions to those rules, of course, but generally that's what you want on a hammer bot. For instance, if you're using a light hammer as a secondary weapon in addition to ram spikes or somesuch (Vetearn Destroyer is a good example) then wedges and trapping capabilities aren't as important.
