Well, first things first. What IS a rub? From Wikipedia.com we get the following.
Spice rub is any mixture of ground spices that is made for the purpose of being rubbed on raw food before the food is cooked. The spice rub forms a coat on the food. The food can be marinated in the spice rub for some time for the flavors to incorporate into the food or it can be cooked immediately after it is coated in the rub. The spices are usually coarsely ground. In addition to spices, salt and sugar may be added to the rub, the salt for flavor and the sugar for caramelization. The simplest rub is just coarsely ground black pepper as in Steak au poivre.
Spice rubs can also have ingredients like herbs, crushed garlic or oil added to make a paste. The spice rub can be left on or partially removed before cooking.
OK, now we know what a rub is, let's look into what makes a good rub. When I am doing pulled pork, I use a very basic rub consisting of fairly equal parts salt, pepper, garlic, and cayenne. Probably about half a part on the cayenne now that I think about it. I do it this way because we give it a sprinkle of seasoning salt as we make up sandwiches for selling and that completes the complex flavors typically found in a rub.
When I do my ribs I use a very complex rub that has a very nice finish. I don't use sugar in the rub itself, but rather apply a rub of just brown sugar prior to my final rub. For other cuts I tend to use a rub that contains some or all of the following in no particular order. Sugar (a must) salt (just a bit, too easy to over do), celery powder, garlic, onion, cayenne, MSG (yes MSG, it's fantastic stuff), paprika (mainly for color), black pepper (maybe white), thyme, oregano, cinnamon (a must in my rib rub), various dried peppers (depending on how much heat I want), and the list goes on.
If you are interested in making your own rubs I would suggest getting a book with lots of rub recipes as a starting point. It will give you a good idea of what goes good together and what spices compliment what meats. And may save you time and money rather than completely throwing out batches and starting over again. Start with something very basic and build from there. You will have your very own signature rub before you know it.
These days there is no shortage of rubs available to the consumer who knows where to look. I usually have a handful of these in my kitchen at all times, because I know how good they are and will get consistant results with them. What's on my counter right now that gets lots of use? Spicewine's Hen and Hog dust, Spicewines Heffer dust, and Plowboy's yardbird rub to name a few. There are others, but I know these guys, their product is good, and I'll get a plug in for them whenever I can.
Applying a rub. Well, there are two types of rubs. Wet and dry. Wet normally is the consistancy of paste to much thinner and you simply slather it on and rub it in. Such as this Walker's Wood jerk seasoning. I was in Jamaica in January and had the pleasure of visiting their factory. It is a great set up. Most of the ingredients come from a co-op of local farmers and they put out some great product.
Dry rubs are just like they sound. A mixture of dry spices and ingredients. In many cases you can sprinkle on evenly or pour it on and rub it in. You can also apply a wet ingredient before applying the rub. I typically don't do this, but occasionally will slop something on there first. With ribs I will use a splash of cider vinegar. Brisket I may use a little terriyaki, a pork butt may benefit from some pineapple juice. These can be slathered on first, or mixed with the dry rub and then applied in essence making a wet rub. Other slathers that I have heard of being used may include mayonnaise, salad dressing, olive oil, mustard or a whole slew of other imaginative compounds. I recommend against the mustard. Some say they can't taste it, but I can taste when it was used and I don't like mustard.
Thanks for looking.