If you buy the HHN addon to your park tickets it's a lot cheaper than buying them standalone, and you don't have to use them on the same day. Weds, Thursday, and Sunday are cheapest and marginally less crowded, but all nights will probably be wall-to-wall people. I always aim to get there 15-30 minutes before the official opening time, they usually start letting people in a few minutes early, and if you move quickly, you can get into two or three haunted houses before the lines get to be an hour long. Try to hit the most popular ones first, those will have the worst lines as the evening progresses. Check the map carefully though, it's not unusual for a couple of them to open an hour or so after starting time.