My camera (canon digital) won't work for longer then a couple of days without saying that it's low battery

You should not be using ordinary alkaline batteries, which will not provide enough power.
Instead, you should be using rechargeable NiMh batteries (the higher capacity the better). Something like 2700mAh.
Or you can use non rechargeable Lithium batteries, such as Energizer e2. They are quite expensive, but will provide you with so much more battery power than ordinary alkaline.
All of the above is applicable if your camera uses AA size batteries, of course. If it doesn't then you need to make sure you just charge the battery more frequently.

Things you can do to increase battery life...
limit the use of the flash
turn the LCD display off between shots (and limit it's use, especially when reviewing photos).
turn off the LCD screen and use the viewfinder instead to compose your shots (if you have one).
limit the shooting of videos
even zooming in and out requires lots of battery power.

Well buy a new battery! Or 2! I found 2 for less than $20 on ebay.

Are you using the flash with every photo? The flash eats huge amounts of battery, so it wouldn't be surprising if the batteries gave in after a short time.

If you're using disposable batteries, stop! You're wasting money, especially if you're buying the dollar-store variety. Rechargable batteries can last for years, and pay for themselves within a short time.

Hello,

If your batteries are old they may not hold a charge for long. You can try cleaning the connections on both the batteries and the charger. I did this when my batteries were quickly losing their charge and it turned out that they actually weren't getting a full charge because of dirty connectors. I used lense cleaners and a file on the battery charger and just the file on the battery. Just rub lightly.

Petra gave you some awesome advice. The only thing I could add (and it sounds obvious) is to be sure you don't leave your camera on for extended periods. Some digital cameras have an auto off feature, when the camera's ignored, but some don't. Also, if you aren't packing spare rechargable NiMH batteries (and you really should) see if your camera lets you turn off the lcd display, allowing you to shoot using the smaller viewfinder. Yes, we're all spoiled by the big bright screen, but it is an energy drain.

I have a Fuji S3000 with the same problem, and those are some of the ways I conserve battery life. One other suggestion is to remove the battery when the camera's not being used. There's always a slight drain, even if the camera's off.