1) Never suggest your friend
There's a reason why a producer booked you and not your friend. It may not be that your friend is less funny than a seagull covered with burnt cigarettes and shame. It could simply mean that the producer has a line-up in mind for next month and does not want to be pressured into booking your chum.
2) Listen to the delivery
A producer may tell you that the show is booked for the next few months. How are they saying it though? Are they thinking about how long it will take till they book you or are they changing the subject to The Dark Knight? Which you saw with them last month.
3) Get there early
If you get there too late, you may find you have been replaced by a comic eager for time.
4) Watch the show before asking for a spot
Unless you are friends with the producer (in which case you don't need tips like this) it's good to watch the show for several reasons. On the producer side of things, you are showing support and, if a comic can't make it, you may get the spot instead. It's also good for you so you get a feel for the vibe of the room. Is it a hipster crowd or is it drunken tourists, etc.
5) Be funny