The traditional approach to flavouring meat prior to dropping it on the BBQ is to soak it in a marinade. If you want the best possible flavour, you may do better with a brine mix.
Amazing Ribs goes into some detail about why brines work better than traditional marinades, noting that the addition of sodium chloride allows the meat to hold onto its juices, leading to a much tastier steak. They're not as keen on marinades, though, noting that they don't sink in very far and may actually lead to burning the surface of the meat on a BBQ. On the brine side, you've got to be careful not to add too much salt — at which point you're no longer brining or marinading, but pickling it. Thanks to Brendan C for the tip!
Salting And Brining Tenderizes, Flavorizes, And Moisturizes [Amazing Ribs]