If the fuse on your power amplifier is blowing, it is important to figure out why. Not only could the amplifier be damaged, but faulty connections may cause dangerous electrical shorts which could result in a fire. If none of the suggestions below seems to pertain to your situation, have a professional stereo shop check out the amplifier.
Power wire grounding
If the amp fuse keeps blowing, the power wire--the wire that goes to the battery or a hot connection--may be grounding out. That fuse exists for just this purpose--to blow before the internals of the amplifier become ruined. It is made to fail so that the more expensive components stay intact.
If the ground wire is not fully connected, or the wire is damaged, spikes could cause the fuse to fail. Trace the ground wire (almost always black for audio amplifiers) to where it is attached to the chassis, inspecting for kinks, tears and other damage. If the wire appears in decent shape, inspect the connection point. If the ground itself is loose and allowed to move in the slightest, it could cause the fuse to blow or the amplifier to shut off.
If the power wire is too small in diameter or damaged, it could cause an abundance of resistance, which creates heat, which in turn makes the amp work harder. The extra heat from the resistance could cause the amplifier's internal safety to kick on, popping the fuse. Inspect the power wire to the amp, and make sure it is at least 10 gauge in size, no matter what the amplifier's power rating is. The goal is to let the electricity flow as freely as possible. Less work means less heat.
Damaged amplifier / wrong fuse size
If the internals of the amplifier are damaged, either through being tossed around in the automobile or being connected wrong, the fuse may blow immediately after power is applied. The only fix for this is to have a stereo shop repair the amp, or replace the unit completely. The amp fuse would blow to protect the electrical system of the automobile and prevent further damage or fire. Also, using the wrong size fuse in the amplifier can cause it to blow. Check the amplifier fuse opening and the power wire--the correct fuse size is almost always listed.