There are a number of natural ways to encourage extra retained fluid to exit. One of the simplest ways is juicing a lemon and diluting it in a few ounces of water (I add liquid or powered stevia to it to take off the bitter edge and make it more palatable) first thing in the morning. It really doesn’t matter what time of the day, but I know for dieters that is usually the best time to take it in. One of the concepts behind retained fluid is a sodium/potassium imbalance in the cells. The sodium/potassium pump in the cell ( to over simplify it) lets sodium hold water in the cell and potassium to let it out. So if you consume high sodium foods without eating potassium rich foods you will create an imbalance that causes too much fluid to be retained within the cells creating water retention.
This is not the only cause however. If the heart is pumping inefficiently or the kidneys are filtering inefficiently or a number of other medical issues are present, you can retain water as well. These need to be ruled out so as not to mask a more serious problem. There is also intracellular fluid that is held for other reasons. There are a number of natural diuretics that are herbal in composition. They usually contain a mix of uva ursi, parsley, buchu, celery seed, green tea, cranberry, dandelion, apple cider vinegar, etc.
There are also a number of foods that encourage loss of water through the kidneys and the foods that work have to do with the cause. In general, foods that are rich in potassium and are detoxifying, such as your leafy greens, are going to help and foods that are highly processed and sodium rich are going to add to the problem. Most health food stores have a hand full of natural diuretic products that are geared at increasing kidney output and increasing potassium and are usually labeled for easy identification.
An experienced employee might know about homeopathics or other less identifiable products that work as well. I would encourage you to identify the cause before you use just any product because chronic fluid retention is often just a symptom of a more serious cause. If you know that you are consuming a lot of processed, high sodium foods and not a lot of fruits and vegetables that are rich in potassium, that is an easier solution. A dietary change is in order. From here- if you absolutely know that you are retaining water chronically for no identifiable reason- it really does need to be evaluated by a health care provider.