you can fix your current strap like Tom above said, but if it is too worn or shot or you want to try something different, look at a Booster or our Dual Strap:
I installed a pair on Andreas Hatveit's boots before Euro-X and he really digs them. Basically it is a 55mm powerstrap that is split in the middle, allowing you to separately tighten the tongue portion and the shell portion. This allows the tongue to sit nice and flush to the shin while separately keeping the lower strap tight against the shell.