To do the mustard trick you can start by preparing the meat to be smoked, patting it dry with paper towels and then coating it in a thin layer of mustard. And best of all you don't have to use expensive mustard. Cheap, basic prepared yellow mustard works great. Now you can sprinkle your rub over the surface of the mustard. The dry rub will stick to the mustard. All you have to do is be a little gentle with the mustard coating to keep from wiping it off. You can wrap the prepared meat in plastic wrap and let it sit over night. The vinegar in the mustard actually works to pick up the flavors from the rub and carry it into the meat. From this point, smoke the meat as normal.
Once the meat is completely smoked and ready to be served the flavor of the mustard will have almost completely disappeared. The mustard actually works to tenderize the meat and you will notice little more than a thin crust over the meat.
Another way to do the mustard rub is to simply add your dry rub to enough mustard to coat the meat. In effect you get the same results as you do with the more traditional method described above.