Try:
1/2 c evoo
4 cloves garlic, pressed or minced
1 small onion, finely chopped (about 2-3 T)
1 serrano chile (or 1 jalapeño) minced, (remove seeds and ribs for less heat)
juice of 2 limes
zest from 1 orange
(or juice from 1 lime, 1/2 a lemon, plus 1 T o.j.)
handful of cilantro leaves finely chopped
1/2 t dried thyme
1/2 t cumin seeds, toasted and ground
1/4 t salt
1/2 t sugar
Mix all together well. Pour over meat in Zip-loc and marinate 6-8 hours.