For those looking for marinades, soy sauce, worcestershire, teriyaki, olive oil, beer, lime or other citrus juices, can be great ingredients. Just remember that if your concentration of citrus is too high and you soak it too long you WILL start cooking the meat. EDIT: I have ruined some beautiful meat before by leaving it in the fridge too long, with too much lime juice.
Garlic, onion (powder, flakes, fresh, minced, or sliced), jalapenos, serranos, salt, pepper, cumin, oregano, and others all work well as part of the marinade or the rub.
If you want to start easy, check out fiestaspices.com and try some of their fajita seasoning. If that gets you into the ballpark you can look at the list of ingredients and start working up your own blend. I have several of their products available in the house all the time, as well as making my own when time permits. Tastes vary, but they have clearly found a combo that appeals to a broad spectrum of customers so that's probably a good place to start.