Marinades or dry rubs and lots of experimentation to pefect your favorite recipes, then stick to them like holy scripture. With dry rubs, the salt acts as the acid tenderizer and works the same as a liquid marinade.
You have to learn how long to marinate the meat with type of marinade/dry rub your using, depending on the thickness of the meat and the acid stregnth of the marinade (or salt content of the rub), and when is too long. For a really tough peice of meat, this could take weeks (corned beef brisket)
Once you figure out how long to soak and how long to smoke, then you stick to that recipe.
Learn to make your own marinades and/or dry rubs. Brines (water, salt, sugar and spices) work really well for dry/lean meats like poultry or deer.
You want the lowest/slowest cooking method that will just barely cook the meat before it begins to spoil. If it's too low, the meat will spoil from sitting at low temps too long.