As a general rule, season whatever meat you're using with the dry rub before you cook it and sauce it towards the end or even after it's finished.
The amount of rub that you use can vary depending on the type of meat that you're cooking, but, generally, you want to coat the meat with rub. If you're someone who likes to be precise in their measurements, use about a tablespoon of dry rub for every pound of meat that you're cooking.
This part is important: Before you apply the dry rub, pat down the meat with a paper towel to remove the surface moisture as much as possible, then apply the dry rub.
As for the sauce, whether you're smoking or grilling your mean, you can baste the meat with sauce as you cook it for extra flavor. But remember, sauce can burn if left on the heat for too long or if the temperature is too high. A great way to baste during the cooking process is with a mop or a brush.
How much sauce to use boils down to one thing: trial and error, but always remember with the barbecue sauce, start out with a small amount and add more as needed.
To recap, always use the rub before you cook meat, let it sink into the meat and sauce towards the end or afterwards for best results. You can even sprinkle a small amount of dry rub at the end for presentation.