They're not really that important. Sometimes you will have girls open you because of them though. I've always loved jewelry, so I naturally have a pretty mean "accessory game".
I love rings, so I wear alot of them. I also have a few bracelets, metal, leather cuffs, etc. As far as necklaces, I have a ton, more than enough to match any situation. They range from a silver figaro chain to string macrame necklaces and a chain maille necklace (most of them I made myself).
By far there are two that I've received the most comments on. One is a titanium pinky ring. Girls tend to ask about it when I wear it by itself. Comments have ranged from "Nice ring" to "Do you think you're a pimp/player?". The other is actually a girls ring that I got from my girlfriends best friend. It's a thin mood ring with little flowers emblems spaced around the ring. Girls tend to notice it alot (I just recently got it) and always want to know the story behind it.
So accessories can be helpful, but it has to be something that matches your personality and style. Just make sure you don't wear that fake "bling" crap I see so many people wearing. If you're going to wear stuff like that, at least get quality stuff. It's hard to look cool when your fake "bling" is leaving a green circle around your neck/wrist.