Wash the meat, dry it with paper towels and season with salt and pepper. Put the meat in a plastic bag and place in a bowl.
Mix the lemon juice, lime juice, orange juice, oil, sugar and oregano together. Pour the marinade into the bag, close the bag, and then put the whole thing in the refrigerator for 4-6 hours.
This is very suitable for grilling, but you can also do it in the pan. Before the meat is on the grill or in the pan, remove the meat from the marinade and discard the rest of the marinade.