Or get and ST-E2... it can project an IR beam to assist focus, too.
I don't have a T1i, but assume it has a "Simulate Exposure" setting for LIve View, the way some of the other cameras do. That should brighten up the screen and make it easier to see what you are doing.
However, be aware that Live View uses a completely different method of focusing, Contrast Detect. It's via the imaging sensor itself, rather than using the array of AF points normally used for Phase Detect method.
Contrast Detect is much slower, though it can be more accurate. I don't know it's low light capabilities, but if you can't see the subject on the screen in Live View, just about any AF system is going to struggle. Thus the need for some sort of focus assist... i.e., an external flash, ST-E2 or the small flashlight that Mitch suggested.
Why are you using Live View, anyway? Though there are times and places LV can be useful, a lot of the time it's far easier and faster to just use the viewfinder and standard AF system.