They say, "Nobody likes a comedian," and this is true. Nobody. But if we've got to kill a couple of hours with some random dude, a good sense of humor is a must. Wait, "Isn't this a giant contradiction?" you ask. No. They are two very different things.
Note we said a GOOD sense of humor. That means no monologues, and definitely no impressions. Little known fact: Rich Little? Worst Wingman Ever. Also, prop comedy is absolutely verboten. Only four people have ever been able to pull that kind of thing off, even professionally. So unless Steve Martin is your bro, you need to avoid this at all costs. Nobody wants to watch your pal explore the improvisational possibilities of a carved out watermelon and an empty case of light beer.
Best case: You guys go home alone. Worst case: You get attacked by the Gallaghers.
Also, it's important to know your audience. While your buddy's belching rendition of "O Solo Mio" may have killed it at the frat house, rest assured, the ladies will not be impressed. You need an entertainer who listens and responds to what's happening around him -- somebody who's good with crowd work, and knows how to warm up a room, but doesn't need the spotlight and knows how to keep things light. Finally, it should go without saying that under no circumstances should there be any kind of ventriloquism. The only dummy at the party should be you.