Ideally you want at least 2 good fans, one on the front of the case and one on the back, place the front fan to suck air in and the rear fan to blow air out, this will cause air to flow through the case continously and will help cool, if you can add fans to the sides then do so, the more the better...
If this is not possible, take the side off you case and place a desk fan blowing air directly into the case, not very nice, very big, but it does work!!!!