Write to your elected officials and tell them global warming is an important issue for you. Write to the newspaper. Talk to your friends and relatives about it; educate them about the realities of global warming.
Live close to your work (or work close to your home). If that’s not practical right now, at least make it a priority next time you’re moving or job-hunting.
Recycle as much as possible.
Try to live as cheaply as possible. Things that cost money, usually cost energy to produce.
Reuse as much as possible
Hah! That’s no problem; I’m a liberal arts student. 
Eat local. It isn’t just you that gets moved around the country - try to buy locally in general, but particularly with food 'cause it tastes better. This is easier in South Carolina than, say, Maine, however. Regardless, in the summer, make an effort to find local produce.
And if you can’t personally plant a tree on your own property, donate to an organization that’s planting trees anywhere in the world. Here are some options to get you started:
http://www.treesforlife.org.uk/helping/pat_life.html
Food & Trees for Africa
http://www.jnf.org/site/PageServer?pagename=Trees
http://www.treesfortheholyland.com/cgi-bin/ccp51/cp-app.cgi
http://www.treesforcities.org/html/whatcanido/giftatree/
I agree. Get a Hybrid or even a conventional high-MPG car. There’s even some pretty high MPG American cars, and even a few SUVs. If every hugefuckinggasguzzler owner sold their guzzlermobile and bought a Saturn Vue or a number of other nice choices, that’d help a lot. In the long term, they even save you $$ (adding in Tax credits, and hoping you drive the car for more than just a few years, etc)
kanicbird said it first- possibly the cheapest and easiest is to switch to flourescent bulbs. It even saves you money!
Odd one- Switch Flashlites to LED only- saves lots of batteries.