Depends on the meat...
You need a base (a liquid)... could be water, could be juice. Depending on the meat, you may want sugar.. You probably want some salt.. Then go to the spices (maybe some garlic powder, cayenne, paprika, dry mustard etc... all your choice here to compliment the meat).. Then you can get daring and add small amounts of things like soy, terriyaki, worcestershire, hot sauce etc... Again... all your choice...
Lots of good recipes for injecting meats... test out the search function at the top using some different combinations of search terms or check the Roadmap thread (sticky thread) at the top of the Q-Talk forum