Often times with fish you want to let the natural flavour speak out for itself, however, you can enhance that flavour in a couple of simple ways, depending on the fish.
Lemon - is like an amplifier for the natural flavour of the fish. A fresh squeeze is best, don't let the juice boil away during cooking.
Herbs - tarragon, chives or parsley go best whilst the fish is cooking (baking/grilling/steaming) but also melted with butter to form a super simple sauce.
Strong Tastes - Horseradish is a particular favourite, particularly with oily/dense fish.
Capers & Shallots - a classic with smoked fish like salmon.