If you're using a paper background, bend the paper at a curve so that it gives the illusion of there being no horizon. It helps draw the attention toward the subject.
Use a tripod. Like really, use a tripod.
Edit the photos! My photos look like crap whenever I download them raw to my computer. Even a little bit of editing helps them to be higher quality. On my computer I use PhotoDirector and it helps a lot, be even before then I used the default Windows Photo Editor and it worked wonders.
Just use the settings to bring up the brightness and exposure, lower the ugly yellow colors that may be in the background, and then brighten up the MOC's colors with the color intensity and contrast settings. Then crop the pictures and sharpen them up (not too much, then it becomes noticeable and ugly).
With these simple things you can go from something like this: