I use a basic marinade of lime juice and zest, orange juice and zest, cilantro, fresh jalapeño, olive oil and a bunch of garlic (salt and pepper of course) for several hours. A bit of Mexican beer can also be used for the marinade.
A trick I like to use for this or stir fry is to dry the meat after marination and dust it in just a very small amount of corn starch so it really gets a nice sear when browning.
A friend of mine will also reserve the marinade, reduce it by a fourth and then spoon a small amount of it on the meat before adding the pico and other stuff.