Although herbal or other alternative remedies are often a great way to stay healthy and treat illnesses, these can actually complicate bipolar disorder. Some studies have shown that remedies such as St. John's Wort and SAM-E, for example, may actually cause people with bipolar disorder to "flip" into a manic state. Also, if you're considering trying herbal remedies, it's important to remember that, just because they're herbal, doesn't mean they won't interact with other medications. So before taking any kind of home remedy, check with your pharmacist to see if they might interact with your current meds.
Most mental health professionals agree that, at least with our current knowledge and technology, medications are the cornerstone of treatment for bipolar disorder. In my own experience working with individuals with this diagnosis, medication really is a requirement in order to stay healthy (and even then, unfortunately, there are no guarantees!).
That does't mean, of course, that there aren't other things you can do that might be helpful as well. If you've noticed a seasonal component to your illness, for example (i.e. if your mood gets more depressed in the fall/winter) you can use a SADS lamp to help increase your vitamin D and reduce symptoms of depression. I've also heard great things about Omega-3; while the research is slim so far, Omega-3 might be useful in reducing symptoms of depression and preventing bipolar episodes. If this turns out not to be so true, however, Omega-3 is also really good for your heart, so you're still doing your body a favor!
Overall, I think the main message here is CAUTION. And if you're considering alternatives, speak with your doctor so that you're making informed decisions.