I work in a health food store, so I might be able to help answer questions. I'm no expert or doctor though, so this is just information and not medical advice!
St. John's Wort has been used for centuries in dealing with depression (or "melancholy" as in ye olde days), and anxiety. In modern studies, it seems to be most effective in mild to moderate depression, especially if anxiety is a cause. It can take two or more weeks to really work, though, like a lot of the modern SSRI drugs. It comes in tablets, although liquid extracts are less likely to have lost potency in general- basically, you can take it either way, so don't worry too much about that. There is also a tea out there by Traditional Medicinals (company in the U.S.) with St. John's in it, and right now I'm forgetting the name of it. The tea has a bunch of other calming herbs in it which work better in the short-term than St. John's does. That way, you get short and longer-term benefits by drinking it every day. Valerian, lemon balm, hops, and lavender are some examples of other calming herbs. Again, as pointed out by shygirl, be careful about any drug/herb interactions (ask your doc or consult a Physician's Desk Reference).
5-HTP is a supplement which has helped me out with depression and insomnia. It is also, like St. John's, related to serotonin (sp?) levels, but it seems to work more quickly. It needs several other nutrients to work in the body, so I just take a multivitamin along with it to help. I think vitamin B-6 is the most important of these nutrients.
None of these things works for everyone, unfortunately. I had to bounce around a little before settling on 5-HTP. It's also more expensive than St. John's, which was a drag.
Ok, good luck.
