Weight gain due to water retention is mostly experienced by women just before their menstrual period begins. The phenomenon is known as premenstrual water retention, and is quite harmless, although uncomfortable and slightly painful. Sometimes however, water retention can be caused by other problems too, such as kidney and thyroid problems, or dehydration, so if your problem is not related to the menstrual cycle, you should get yourself checked by your doctor.
One simple measure that works against fluid retention is decreasing your intake of salt. Salt causes the body to retain water, so simply decreasing the amount of salt you consume in the week before your period can help quite a bit. Also, contrary to what you might think, drinking water helps too. As mentioned before, dehydration is one cause of water retention. Rather than adding to the build up of water in your body, drinking water will help flush out excess salt from your body and thus minimize fluid retention. Exercise also helps, in particular swimming, as the exertion improves body functions and the pressure of the water helps push the built up water from your tissues.
In addition, avoid alcohol, tobacco, oily food, and tight clothes, and raise your legs as often as possible.
answered by M W