OK, so you've got your flash head unit, an amp or two (or three) powering multiple speakers and subs, but what have you done to the electrics?
One simple and (more importantly) cheap upgrade is something known as "The big three".
It involves replacing a few factory cables (3!!) with something more substantial.
1. The ground wire between the battery and the car chassis. Replace this with at least a 4awg gauge cable.
2. Change the charging wire between the battery and the alternator, again with at least a 4awg cable.
3. (Only applicable if your car has one!) Replace the alternator chassis ground cable with 4awg cable.
See, that's not too bad is it?
Doing this helps minimize voltage drops caused by high resistance in the stock cable, less voltage drop = more power for your amps!
One simple and (more importantly) cheap upgrade is something known as "The big three".
It involves replacing a few factory cables (3!!) with something more substantial.
1. The ground wire between the battery and the car chassis. Replace this with at least a 4awg gauge cable.
2. Change the charging wire between the battery and the alternator, again with at least a 4awg cable.
3. (Only applicable if your car has one!) Replace the alternator chassis ground cable with 4awg cable.
See, that's not too bad is it?
Doing this helps minimize voltage drops caused by high resistance in the stock cable, less voltage drop = more power for your amps!
Ive copied this into the Power issues guide too where this subject was lightly touched on IIRC 
