Need a little more information. Why do you think your charge is low?? If you are sure your charge is low, then there is, more than likely, a reason for it being low - a leak somewhere.
Is your compressor coming on when you turn the AC on? You can watch to see if the AC compressor clutch engages when you turn it on. It's possible your compressor is not engaging and, thus, you can't get it to charge. You might have a bad clutch. Or, the charge is so low that the low pressure switch will not let your compressor engage. You may also have a faullty low pressure switch - it's a very easy fix.
You can use a UV dye to help identify the leak. Are you trying to charge on the high side by chance? You have to charge from the low pressure side while the car is running and the compressor is engaged. You need to take pressure readings on both the high and low sides to make sure your pressures are in spec.
There's usually a little more to it than "adding a can or two."
Be very careful when charging your AC - bad things can happen. Always use proper safety equipment, including eye protection.
Hope this helps a little.