This may seem obvious but make sure your ISO is set at 1600, or 3200 if need be
The Canon 450D isn't capable of ISO 3200, and 1600 has so much noise it's useless. (Seriously, 800 is pretty easily salvaged as long as you exposed correctly, but 1600 has this extreme banding in the shadows that can't be removed by any software I've tried, including LR2's noise reduction, Nik Define, and Topaz DeNoise. DeNoise was able to remove the noise at a severe sacrifice of sharpness, so the only thing to be done is put it through Simplify and see what happens.)
My advice is, whatever you do,
don't shoot at 1600. I can't stress this enough. Go to 800, and invest time (and possibly money) in good noise reduction software. Make sure you get your exposure bang-on or the noise will be incredible and make the shot useless.
So, without the safety net that a higher-quality camera provides, you're going to have to get creative, and I dare say use some more skill. (This is the point where I note that weilding these entry-level DSLR's and still getting good shots in crap conditions requires a fair bit of skill over a top-end camera where one can just pump up the ISO to a bazillion and get a shutter speed of a billionth of a second.)
Practice stabilizing the camera. If you have a tripod, and can use it at the event, practice panning with it. Just make sure you have a really good feel for it and the lens. Practice your hand-holding, A LOT. In all likelihood, you're going to have to go out of the safe zone of 1/focal length for shutter speed. It's entirely possible to get a sharp shot at these slower speeds, but you have to be relaxed, steady, calm, and practice every single technique to stabilize the camera that you can. Holding your breath before squeezing the shutter. Positioning your arms properly and posture. Dig up every tip you can on this; if you can master it it'll save your butt.
Start getting comfortable with manual focus. The AF Servo on better bodies is great; on the 450D it's less than stellar. It won't track your subject very well while their moving (and in low-light conditions, you'll be lucky if it tracks at all). So start using MF for, well, everything, and get really comfortable panning and focusing on moving targets (while observing all the above technique to stablize the camera).
Try to use the lighting to your best advantage. Observe where the light is strongest (try spot metering various places in the arena), and time your shots for where the light is most intense, which will help you when the camera calculates the shutter speed (based on your f-stop; I'd recommend just going with Av at f/2.8 and letting the camera sort-out shutter speed).
Or, if all that looks like it's going to quickly become a nightmare (which it really shouldn't; film shooters not so long ago had to deal with situations just like this, and still got good shots), rent a higher-end body, like a 40D. It's the only way you're going to get ISO on your side.