Andrew Robertson , Feb 19, 2006; 08:27 p.m.
The first thing to try is setting the AF to all points, and then setting the lens barrel switch to 'M'. Press and hold the shutter half way down while manually focusing the lens, and you should see the AF points light up in the finder when the area covered by them is in focus.
This is often a really handy way to focus, especially in lower light.
Another thing to try is the angle finder. You can zoom in on the finder and get much finer grained focus. This is an expensive one, but I have used one for a while and am going to get one because it's nice for tripod work too.
Finally, there are at least two individuals who manufacture and sell their own custom made split prism focusing screens on the internet. I am not aware if they have them for the XT, but if not you could get one for the 10D or 20D and cut it down. I think they run about 40 bucks.