Rich,
The most important thing about touch-ups is to use quality paint for the touch-up. Unfortunately, most people do not have that lying around the house. It is next to impossible to get quality paint in small quantities, although with white it won't be as difficult. Some of the tricks to good touch-up work is to do it in the proper steps, including surface prep, primer id neccessary, layering of the color, sanding, clearcoating, and rubbing out. There is a lot of work if you go all the way. Sometimes an airbrush can work wonders, but only if done in the proper steps.
If you want to use hobby paint, reducer will help prevent "blobbing" and aid in a smoother result. Clear over the top and some rubbing out will help. Make sure the paint is VERY dry before doing any sanding or buffing.
Hope that helps somewhat.
Brian Baylis
La Mesa, CA