I HATE using dummies, but I do it anyway! Like you, it cost too much to practice on a real cake, and frankly my whole family is sick of cake! LMAO!!! First off, you have to have something on the dummy the fondant can stick to...such as buttercream. make the buttercream as the same color as your fondant, that way when you take off the fondant right after you take your pictures you can just knead it back into your fondant....You can also use crisco to cover the dummy and the same thing applies...Just knead it back into the fondant.
Biggest challenge in doing dummies, is getting them to stick to your cake board!!! If it ain't stuck, you're gonna have a hell of a time. I have found that rubber cement works the best! Just give it time to dry and it will peel off your dummy with no problem when you're done. Scrape off excess buttercream or crisco, then wash in hot soap water, dry and put away fro future use.
Hope this helps