Image clarity & sharpness on shots, especially action shots

I have the Canon Rebel XTi; am just starting to use it for action shots - equestrian events. The lens is the standard "kit" lens that came with the camera (18-55mm). It's difficult to find specific info as it doesn't seem like a lot of people out there photograph horses. The problem is the image of the horse (and rider) is just the slightest bit out of focus and also just a bit dark. The background comes out fine, but the actual subject I am centering on is just not up to par in the sharpness and clarity I am looking for. This seems to be the rule whether the animal is standing still or moving. I have been playing around with the shutter speed, aperature setting, ISO setting, etc. but seems to be the same result - also the same with the auto settings on the camera. I'm being really careful about the focus points, etc. I am guessing maybe it's a shortfall of the lens itself? - from what I've read about the 18-55 lens it doesn't seem to be really good for much of anything... : (
Any advice for things to try? Along with getting a new lens? Thanks!

Just as an experiment to see if it makes any difference, you could try using the same techniques you are now using except put the camera on a tripod and control the shutter with the self timer. this will eliminate any motion caused by the operator's hands. If it solves the problem you then know that it's not the lens and have saved yourself the money of a better lens. If it makes no difference, then your next step is a better lens.

Hi. Sorry - I do have a tripod; forgot to mention it (great suggestion, tho)! And even holding the camera I am going on the assumption that it's not camera shake since the background is clear (ie - the jumps, the ring fence, anything else basically!). I have, since I posted my question, found some more info. And tried some of the other settings in the camera (metering mode, adjusting sharpness, etc.). That has helped some, but I'd like for more improvement. It's appearing that the advice, with that in mind, is going with a new lens. It was mentioned a 70-200. I've also seen a 70-300. Suggestions? I'm a rookie on lenses so know nothing about them! I've been told Tambor (?) is a good lower cost alternative to the Canon lenses. Thanks again!

Hi. I don't think the lens is the problem. Try to set up your camera focus point and metering mode and test it with all settings. Best for action is to use a 5.6 - 8 aperture (with kit lenses even F/4 is ok if the distance from subject is longer), exposure metering to centre and autofocus point anything but not all points. Also try manual focus (focus without subject to where you think that he will be).
About the lenses for action / sport, 70-200 is expensive but very optical good quality, 70-300 is cheaper but not so good in optical quality. Also, try to get some lenses with ultra-sonic motor - they focus faster and you also have full manual auto-focus all the time. Tamron is optically ok, but their focus system is slow. For budget lenses, I recommend Canon EF 70-300mm f/4-5.6 IS USM (about 500$), as for professional entry level I recommend Canon EF 70-200 F/4L USM (about 600$).