How do I make myself become more qualified for a graduate assistantship?

I am getting closer to the end of my undergrad degree, and I noticed that I may not be able to afford my grad degree, due to the fact it would require me to move. I have heard that graduate assistantships (TAs) can really help to pay for school and living expenses and I was thinking that this is my best way into grad school (aside from fellowships).

So, how do I become more qualified for an assistantship? Do I need a certain GPA? I have volunteer experience teaching, would that help? What can I do NOW to make myself more qualified when the time comes?