You should not sand your rims. All you'll do is wear away some metal and the pads will soon have the surface right back where it is right now.

If your brakes have gotten poor over the past short while it has to be due to something that is interferring with the brake arms. On V brakes if you're using fenders be sure to check that the arms are not hitting the fenders.

Another possible issue may be that you ran your wheels and pads through puddles with an oil slick on it and your rims and pads got contaminated. So clean the rims again with a strong dish or laundry detergent and also clean the pads well to remove any oil.