I usually make my rubs on the day of the rubbing. Basic starting point is always salt and brown sugar. After that, it all depends on the "flavour profile" (oooo...I watch too many cooking shows!) that I'm going for. My run-of-the-mill mix always includes a LOT of coriander, a good dollop of cumin, and ground chipotle. Tossing in a bit of ground coffee never hurts. Oh... and add a pinch of baking soda (not powder). Odd you say? The slightly alkaline rub will promote the Maillard reaction when the meat is getting browned.
My best recommendation is "mix and taste". Like your finger, dip it in the mix, and give it a taste. Adjust as needed. If it doesn't taste salty, you need to add it.
When it comes time to salt the meat, ALWAYS dry the meat. You don't want a slime on the surface. Wrap tightly with cling wrap and then into a zip lock bag. For red meats, 24 hours in the fridge
is best. 4 hours for fowl. When you are ready to grill DRY OFF THE MEAT. The rub always draws some liquid out. If you don't dry the meat, you are going to start off by boiling the liquid on the meat instead of going for a crust.