When you make your egg mix, use cream, not milk and dont be afraid to use a lot of cream to the amount of eggs. Makes a creamier and smoother texture and not so eggy.
Asparagus, tomato, ham, blue cheese, peas are all good too.
If you use spinach, and cook it before hand, make sure yu let it cool and them squeeze the hell out of it to get rid of the excess water, otherwise your mix will never set and if it does it will be slimey and watery.
I use shortcrust pastry and put it in cupcake pans to cook in. The trick is low and slow...cook them at about 170C, they may take a while, but they will be less dry.

HTH GL