Like Allison said, it's difficult to give specific advice without knowing what you've tried so far.
I can offer up some generic suggestions, though:
* If your calorie intake is just barely enough to lose weight, you could lower it - a LITTLE; too low too fast can be a bad thing!
* If you've already cut your calories to a fairly low amount, you could consider trying the "zig-zag" dieting method. Some sources recommend you eat 3 days at your low calorie level, then one to three days at your "maintenance" calorie amount or a little higher, then back down to the weight-loss calorie level for 3 more days. Like if I were to try it, I'd eat 1500 calories Monday, Tuesday, and Wednesday, then 1800-2000 calories Thursday, then 1500 again for Friday, Saturday, and Sunday, etc. It's supposed to keep your body from getting too used to its calorie deficit and lowering your metabolism to compensate. Our bodies can sometimes be more adaptive than we realize!
* Add in weight training to your exercise plan, if you aren't already doing this. If you're not losing, maybe you can take advantage of the surplus calories to build a little bonus muscle, which should by default increase your basal metabolic rate just slightly. Who knows, maybe it would be enough to nudge you off your plateau.
* The last suggestion (and what I would probably try first if I hit a plateau) is changing your exercise type. If you usually walk, try stairs or jump rope or get on one of those crazy gym machines to get your cardio for awhile. Some folks say it's a good idea to keep your body guessing, or it can adapt itself to most any routine and make itself a more efficient, calorie-conserving survival machine.
All of this info comes from my personal research online, in books, or from these very forums - I haven't been losing long enough to hit a plateau myself, so I'm afraid I can't provide personal experience. They sound reasonable to me, though!
I still do recommend posting your current plan so our resident experts can give you more helpful and specific advice. Either way, good luck!