Booze is OK but it's short term relief for a mid-term problem.
Some of the things I've done are:
1) Find a good sense of humour ideally with a certain amount of charisma
2) Get some good, harmless in-jokes going
3) Do a funny award ceremony
4) Send out some amusing Dilbert cartoons representing some of the current difficulites (www.dilbert.com has a great key-word search)
5) Drop a couple of mails to the senior partner every now and then to "recognise" key individuals or, better, get the partner to do it
6) Sit down, in the tough times, and get your hands dirty in the menial stuff (test, draft, format) - whatever the project is
I've also - in really tough times - bought Amazon vouchers from my own pocket for some key guys who really gave a LOT up for the project. Depending upon your project you can charge them back....