Check out the latest research, published in the February 2010 issue of Journal of the American Sexually Transmitted Diseases Association, that supports the fact that douching is not only unnecessary, but it really can be bad for your health!
This particular research provides more evidence that the association between douching and Bacterial Vaginosis (BV) is causal (meaning that douching seems to cause BV, rather than women douching in response to already having symptoms of BV). The authors of the study concluded that "douching, but not other feminine hygiene behaviors, is significantly associated with BV."
There is a brief article about this on the Society for Menstrual Cycle Research website.
When you douche, good bacteria (Lactobacilli) that protects and regulates the balance that keeps your vagina healthy are removed, allowing any harmful bacteria that may be around to multiply in an uncontrolled way. And douching, as well as the presence of an untreated STD (or any irritation of the vagina or vulva) or a significant lack of good, healthy bacteria can increase the likelihood and the risk of contracting another STD including HIV, if exposed.
Anal douching, for many of the same reasons, is also unhealthy and not recommended.
But, I can't write about douching without acknowledging that there are people for whom douching might be a part of what makes them feel comfortable sexually and hygienically... if you are one of them, I still recommend trying to avoid it or doing it as infrequently as you can. But if you are going to do it, definitely don't douche immediately before or after a sexual encounter. In fact, I would recommend waiting several hours or even overnight to have sex before or after you've douched, to allow your body time to build its natural defenses up again. And if you are having sex after you've recently douched, definitely be sure to use protection properly.

Allegra---Thanks for the great blog and post...and thanks for including the issue of anal douching which is an important issue for many queer men who bottom. Indeed there is a lot of evidence that not only is douching not always necessary, but that it can be harmful and increase the risk for catching sexual infections by washing away health protective cells in the rectum. So gay/bi men out there----listen up! This article isn't just for the ladies! *smile* I also love that you've acknowledged that for many, even understanding the health risks, douching is something that they feel they need to be entirely comfortable with their body (sexually or otherwise)....and in addition to the tip to not to do so too frequently...we should also mention that if you are going to douche (especially rectally) NOT using large amounts of liquid and using ONLY lukewarm water are also important harm reduction strategies!
thanks for the additional information, Erik!
yes, definitely, guys (and ladies!) who are anally sexually active: if you must douche, using a SMALL amount of warm water is imperative, as using large amounts can be dangerous and damaging to your digestive tract...
and NEVER use a vaginal douche for anal douching!
... and i actually should add that you should never really use a vaginal douche for vaginal douching, either! warm water is ideal.