Matt 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. The ground wire between the battery and the car chassis. Replace this with at least a 4awg gauge cable. Change the charging wire between the battery and the alternator, again with at least a 4awg cable. (Only applicable if your car has one!) Replace the alternator chassis ground cable with 4awg cable.
Matt I remember it from when I used to spend a lot of time on Talk Audio when I first got interested in cars. I've never done it, but I suppose it one of those "every little helps" things once you've got the stereo system sorted.