I take Omega-3 supplements and Vitamin D. For Omega-3, you need to make sure that what you're taking has a high concentration of EPA fats, as these are the ones that will actually help with mood stabilization. Look for capsules that come from fish oil, not flax seed, because flax contains a different type of Omega-3 (ALA). I use a brand called OmegaBrite because my doctor recommended it, actually. Apparently it's one of the best out there to help treat bipolar disorder and depression. I'm still always kind of skeptical about this sort of stuff, because there are a lot of companies out there who are taking advantage of this Omega-3 craze by marketing products that aren't really going to help at all. I'm trusting my doctor on this, but in general I think a person looking to take fish oil just needs to read up on the supplements being bought to make sure that they're going to be beneficial, not just a waste of money.
Apparently you do not want capsules that contain Omega-6 and Omega-9 oils as well as Omega-3. You can always boost Omega-3 intake by eating more "fatty" fishes, like tuna and salmon, but I think for bipolar disorder we'd have to be eating fish around the clock, and there's always that whole mercury content thing to worry about, so supplements might be the best bet in this case.
You need to make sure that you're taking a high enough dose to see any real benefit. Since this isn't really regulated like a pharmaceutical, individual doses are going to vary a lot. To see psychiatric benefits you need to be taking a much larger dose than a person who takes it simply for general health needs. I've read things like minimum 1000mg, but it could be a lot more than that. I'm taking 2100mg right now, 6 capsules at 350mg each.
Personally, I do think that it's helping at least a little bit, although I started taking it at the same time that I did TMS therapy, and when I finally increased my Lamictal to the actual therapeutic dose, and when I was able to get out of an extremely stressful environment and into a safer and less stressful one... haha, so all of these factors came together, though I'm not sure which one did the most work to get my stabilized!
As for the Vitamin D, I don't know too much about it, though I'm sure some of the same principles apply, like making sure that you're taking a concentrated form at the right dosage. Bpchrisb started a thread on this a little while ago that might be of some help:
http://www.medhelp.org/posts/Bipolar-Disorder/Vitamin-D/show/1190494
Good luck!