Rubs

I would like to make my own rubs. Are there any general principles to follow ?

There are some rub recipies in Steven's books. Sometimes I just like to take some already made seasonings and slap it on and let it sit in the fridge overnight, or you can put it on right before grilling.

I've made my own that I've culled from a variety of sources. As a general rule though, I'm guessing you will want sugar (I use dark brown - for the molasses), salt and spices -cumin, paprika,black pepper and chile pepper are the most common. Try experimenting to find flavors that you like. I added garlic, onion salt and others to mine. Try different herbs or more unique spices. Experiment with the type of chili pepper that you add. I've used chipolte chili pepper which adds a great smokey flavor to the rub.
In the end though it is generally sugar, salt and spices. The sugar helps to carmalize a nice crust on the meat, the salt does what salt does and the spices are up to you.
EXPERIMENT, Have fun!