For chicken breast or pork tenderloin, I use evoo, crushed garlic, white wine, lime juice, freshly and coarsely ground red pepper (not chili, but proper peppercorn with a bright crimson colour. They have a wonderful distinct aroma, but if this can not be found, black pepper, also freshly and coarsely ground would do), a dash of dried herb mix for grilling and a drop of stubb's hickory smoked flavour. (not necessary if you are using a proper BBQ grill to cook the meat)
When I do the prawns, I don't bother with marinade, I just grill them with evoo, garlic, pepper and a squeeze of lime or lemon.