buy the best enginered models you can, such as Tamiya's. That will usually reduce the fit problems. There are VERY few models, and none that I've built, that will give you no seams, gaps to be filled in a way or another.
To ease up on sanding, use a Q-tip with Polish Nail Remover to get rid of the excess dried putty. Or use epoxy putties that can be worked with water while they dry (Milliput).