I agree with Stormness_1, for the most part it's not the design or quality of work but the economy that is the problem. You will find the right buyer eventually.
The only real advice I can give is to only try to create customs you love so much that you won't mind keeping them, even for a short while as you're waiting for the right buyer. Of course I know the situation is a bit different for someone who makes several customs each month than for someone like me who rarely makes more than ten per year... you may run into storage and display problems even if you'd like to keep them.
I'm with puuush, it would be sad to bait a pretty pony just because it didn't sell. If there's something about the design that bothers you, try changing that and trying again. Or maybe simply have a new photoshoot, try different angles and backgrounds (if you prefer them over simple white). Finding new venues for selling is also worth trying - Etsy, your friends and family, conventions and meetings and all sorts of happenings you can think of... Sometimes people (esp. non-pony people) are more inclined to buy if they see the item in person so that they can truly appreciate the effort and skill that went into making the piece.