very good question - I would suggest maybe adding a small depression under where you want to put the greeble - take a drill bit, and give it a twist against the plastic. That way, the glue has a place to go when you push the two parts together. Also, use as little glue as possible...
But, this brings up a secondary, similar question, that I've been curious about - say you've got two pieces of flat styrene, both of them the same shape, but one has triangular openings cut into it, so once they are glued together, it looks like a frame or the inside of a piece of armor with some internal structure. How do you glue the two pieces of styrene together - do you put as little glue as possible, and try to clean up the squeeze out as quickly as possible? Do you let the squeeze out dry, and then cut it away?