I tend to work with venues I know and have built a relationship with, therefore when it comes to negotiating, your already on the right page. Be polite, honest and transparent about the budget and see what they are flexible on to get the better deal for your client. All venues have wriggle room one way or the other and if you dont ask you dont get. Always offer the potential of repeat business from the client and or yourself, preferred supplier status and building that relationship that is a win win for both of you. Remember they want your business more than you need their's, that's if you have sufficient time to play with.