I have 3 of my tanks with the backs painted.
I didn't do any real specific prep work, I just cleaned outside real good with glass cleaner (none got inside) and then I masked it off completely, including the top, sides and front, I didn't want any overspray, especially inside the tank. It took several light coats, but it turned out very nice. If you have any "hard water" marks, you have to scrap them off real well, as the paint doesn't cover them very well (be careful, you can scratch the tank while scraping).