An Irishman once told me that only Irish-Americans wear green on St Patrick's Day. "It's a serious, religious holiday over in Ireland," he said. "They don't even know what corned beef and cabbage is!"
I don't know about that, but I don't see any harm at all in donning a bit of green on March 17th, just as long as it's not head-to-toe. Then again, if you're attending or marching in a parade that day, anything goes.
For those of us who are more inclined to be reserved, there's always the lucky charms opt
ion:
This is a gold-over-sterling four leaf clover charm with cubic zirconia accents. You could wear this on your bracelet even after March 17th. 
Do you want something more traditional? Then choose a claddagh ring like this one.
Of course, if you want to wear green bling on St Patty's day, what better way to do it than with genunie emeralds? Check out this circle pendant:

That's just the tip of the emerald iceberg, of course. Find more at at Holsted Jewelers.
Happy St Patrick's Day!