My top tip would be make partents leave!! I know this seems initially v scary but beleive me they are better behaved without them and you feel you can take charge and give directions more. With the parents there you feel they should discipline their own children, but often they don't and you are left silently screaming.
Also I agree that constant games and activities are ESSENTIAL. A free for all is wild and chaotic, and I actually don't think the children really enjoy it.
Too many children and you can't play games properly. The least we've had is 8 the most 20. At the last party I had a whistle which worked well ( it was a world cup party). We always have a theme. We've had superheros, cowboys and Indians and world cup. Ds1 is now 6 and next year wants just a trip out with friends. Suits me as now ds2 turn. First party at 4 and he wants a pirate party. Dh and I also dress up which the children love.
We've had one party in a hall which was great as the house was not turned upside down. But I think I'm old fashioned in an Enid Blyton sort of way and I love them around the table at home with party hats on, and lashings of ginger beer!!
Totally agree on the evilness of party bags.