Humming works well for a warmup, actually. Hum different notes throughout your range and hold the individual notes for awhile. This has the added benefit of making you less 99.999% less gay than most vocalists during their warmup routine.
The older I get, the truer some very old singing advice becomes to me: namely, relax. I spent 15 years in a band screaming at the top of my lungs, and if I had to do more than 3 shows a week, it wasn't going to happen for that 4th show. I'm 41 now, and while my range should be less than it was in my 20's, I find the opposite to be true. As long as I relax, I can cover more ground more accurately.
So I guess what I'm saying is try humming and smoking pot.