I've been about 5 times over the past few years. I've done the Friday VIP thing and the past few times have gone Sunday afternoon. Unless you are looking for specific, hard to find albums, I don't think paying extra to go on Friday is worth it. Friday is mainly collectors and other dealers who know what they want and are flipping through crates as fast as they can looking for it. Given what you're looking for, you'll find a lot of good stuff on Sunday. Also, there's a lot less haggling on Friday than Sunday.
My advice is to take 30 minutes or so when you first get there to get the lay of the land before you start digging through crates. Some vendors are going to be a lot less expensive than others; some are going to be more organized; some are going to have a lot more albums; and there's some that hit on all three. Other vendors will specialize in hard to find albums while some will have a more mainstream selection (relative to the genre). Some guys are in it to make money, and others are hobbyists who do it for fun and to fund buying records for themselves. The last thing you want to do is to waste time and energy going through some guy's disorganized crates of expensive albums when you could go to another guy's booth whose records are in alphabetical order genre and less expensive. (Made this mistake before)
Also, don't be afraid to lowball the fuck out of people.