How do you grill Shish Kabobs so they aren't so chewy?
Using a cut of meat without too much connective tissue will definitely help. Talk to your butcher about good options. Stew meat isn't the best choice; it contains a lot of connective tissue which breaks down during the long, slow stewing process. Grilling is too hot and quick to break down that tissue. If you do grill stew meat, lower temperature and longer cooking time may help some, but in that case you'll probably want to do the veggies separately.

A good marinade will help. You can add some meat tenderizer, or make fresh pineapple juice one of the marinade ingredients. Pineapple contains an enzyme that'll break down some of the connective tissue, but it has to be fresh -- canned juice will have been heated, destroying the enzyme.