Are you talking about a commercial case that you purchased, like Otterbox or Body Glove? If that's the case*, then you simply choose a corner where the case and the phone meet, put your fingernail underneath the case and start wiggling. It's a trial and error thing but in the end, you're not going to damage your phone, no matter what you do. If you're careful, you should be able to get the case off with little or no damage - but be aware that the more times you take the case off and put it back on, the looser it will fit. With the body glove case on my DX, after 3 or 4 months of off and on use, the case simply fell apart one day. *pun intended
-Mike