Don't overdo the marinade!As a rule, poultry and seafood are not tough cuts and could turn to mush or leather if left in a tenderizing marinade for an extended period. In fact, fish can be "cooked" in acid, requiring no heat at all as in one of my favorite dishes, Ceviche. Extended marination of tender seafood can actually toughen it by "overcooking" it.
One-half hour of marination time before cooking should be sufficient to impart the flavor of the marinade to seafood. Marinated recipes that will not be eventually oven-cooked may specify a much longer time. Thirty minutes to one hour is usually sufficient time to successfully marinate poultry.
Using leftover marinadesIt seems a shame to discard that flavorful mixture, but do not be tempted to reuse leftover marinade without first cooking it. During the contact with raw foods, the marinade most likely has picked up harmful bacteria that could make you very ill. For the same reason, it's wise to cook leftover marinade before using it to baste with.
Frugal cooks can put the leftover marinade to use as a sauce, but it must first be boiled for five minutes to destroy any harmful bacteria. Of course, this boiling process will render it useless as a tenderizing marinade, but it can still impart some flavor as a sauce. Alkalines leaked from the first marinated food will interact with the acids to diminish the sharpness or acidity of the original marinade.
More About Marinades:
The science of marinades and how they work
Natural marinades and flavors
Leftover marinades and marinade tips
Marinades and health