Keep hybrid characters to a minimum. Vulcan/humans, bajoran/cardassians, Borg/Magnavox, whatever. These types of folk should be rare.
I'd add to that and say if you are going to have hybrid characters, then have it two species maximum
Have a Human/Vulcan individual, not a Human/Vulcan/Klingon/Bolian--after a point it just gets ridiculous. If you want a blue-skinned, lumpy foreheaded, pointy eared character, then create a new race to fit the bill.