Well, to really beat the stuff from the grocery store - you should also start with ingredients better than you can get from the grocery store...
The other problem (as I see it) with home made is that unless you go through your home made powder pretty quick, you've got the ingredients (which generally must be bought in quantity) sitting around going stale.
I've found the solution to both problems to be Penzeys
. They sell high quality ingredients at reasonable prices, and frequently in small quantities. They also sell some dang good pre mixed powders. I'm particularly fond of their Chili Con Carne seasoning, chili flavor with almost no heat, which lets me tune the heat and flavor levels separately.