Unless base defense or long range, generally try 75% fuel -- 25% for alt and getting to the fight, 25% for the fight, and 25% to get home on.
Naturally this varies so much with the different planes it's more an easy ideal than an actual practice. However, it is a pretty good basic survival plan for most aircraft.
Tried drop tanks when they first came out, but unless need more than 100% internal fuel, would rather burn all internal fuel and avoid the drop tank drag.