Plant trees so densely that the enemy pathing goes all screwy, especially trolls & large creatures. Works well when you combine this with a raised grounded area, built on top, & spread out your work stations & crafting buildings strategically around your base for your build- some of them are able to block spawns all together.
Also:

Originally posted by Kyle:

I built on a small island and released wolves outside of the walls. They naturally breed without help and have since spread to the mainland :)

This helps too, so much. I did the same thing on a large meadows/black forest island Mine are a bit out of control at the moment though, won't help with lag at all, but they breed & self sustain off the lower tier mobs that spawn 24/7.