We just got back from FL and visited WWOHP on Sunday the 8th. I read so many tips before we went that I just had to post our experience. We stayed offsite so we got to the park around 7:45am on Sunday morning. I had read that Sunday morning seems to be less crowded and it certainly was. Went left towards Marvel Island/JP once we got into the park. I was nervous at the swarms of on-site guests feeding right into HP but there wasn't much we could do!
We followed the masking tape line through JP to the Discovery Center where the line began. Arrived there around 8am and were the fifth group (or so) in the line (I was happy with this!). We were let in at 8:30 and went right to FJ. Line said 45min but we would only have waited 15 at the most if the ride hadn't broken down when we were inside the last holding room (!). As it was, we waited 20-25min and got on the ride. We did child swap which was great bc we didn't have to worry about storing our bags. Child swap worked really well with our 4-year old and our 6-year old got to ride again with dad.
Got off the ride into a packed Filch's and headed right to FOTH which only had a 5min wait. Great ride! After that, bought wands at the wand cart for our three boys. Didn't see the Ollivander's show but have to say that even the wand cart was pretty awesome in the way they let the boys choose, try out, examine their wands before buying! Great experience. Rain started right then which mobbed the stores even more. We hung out a bit in a side alley behind Honeydukes before venturing into it and Zonkos. The stores were SO crowded we just decided to leave the HP area and venture over to other rides to get out of the rain. I was worried we wouldn't get back into WWOHP once we left and the day wore on but because of the rain we were able to wak right back in around 1pm! Got the rest of our souveniers (line for Dervish & Banges was about 15min), took more photos and felt like we did everything. Dragon Challenge was closed for a long time due to lightening and our kids were too small anyway so this was no big deal.
Other than all the stuff written before me about getting there early, stowing your bags in advance, etc., my only other tip is that the Universal store OUTSIDE the park has almost all of the HP merchandise (except for wands, I think). Buying the t-shirts, etc. inside WWOHP was neat but so crowded!! Wish we had known that we could get much of the same outside the park without fighting immense crowds.
Last thing-- the clerk at D&B said it was an EXCEPTIONALLY light day that day due to it being a Sunday AND because of all the heavy rain. Counted ourselves lucky for that

Good luck to everyone else. It was definitely worth all the planning and waiting. WWOHP is awesome! Can't wait to go back and hopefully see even more next year.