It depends on what kind of weight it is, and how off the fit of the dress would be. You could try using an over-the-counter diuretic from the diet section of a grocery or drug store to rid yourself temporarily of excess water weight, but you should avoid them for long-term use. There can be some unpleasant, all the way to dangerous side affects depending on how long you use them, so read labels, do research, and try out the pill before you have to wear the dress to make sure it isn't going to cause any adverse reaction and that it is even going to help.

The other option you have it to take it to a tailor and see if they can take the dress out a bit to make it fit. It can vary in cost, but it will likely be more comfortable and about or less expensive than just about any other alternative.

Shape-wear is also an option, but is likely an uncomfortable one.

It the weight-gain was sudden, talk to your doctor asap! It can be a sign of a serious problem. Also, while you are there, you can discuss diuretics.

I did WW for a while, and the first week you drop a lot of water weight. While it is just water weight, it might do the trick. Drink a lot of water, eat lots of fruits, veggies, etc. And, keep an eye on your portion control.

But, like Ashley said - if this was a sudden wight gain, not do to holidays/parties/something similar, you should talk to your doctor.

Also, Spanx work wonders :-)