The next important item on your list is props. You can't have a lip sync battle without microphones (I purchased a pack of toy ones off of amazon) but you also need fun props for your lip-syncers and the crowd to have too. I went to the dollar store and just walked around and grabbed random items like bead necklaces, glow stick bracelets, glow stick clappers, noisemakers, crowns , feather boas and so much more. I also purchased a 12 pack of sunglasses for everyone to grab while performing or taking pictures.
Lastly, what's a party without party favors? Everyone got one of these party favors, because everyone participated, those who chose not to lip sync got to be the judges to determine who won the award that I bought for the winner, which wasn't the greatest of awards but it was still pretty fantastic.