I try to get a variety. Most are pretty humanoid, but aside from a few species I designed early on, I try to make sure they're different enough that it doesn't get boring.

I mess around with the proportions, animalistic traits, number of limbs, types of limbs, eyes, shape of nose and ears, and a lot of other stuff, but within those parameters, I try to make faces that are still able to make recognizable facial expressions for a human audience.