Sound system draining battery on 96' Chevrolet pickup?

I have a 96' Chevrolet pickup ext cab that I just put alot of speakers into.. I replaced all stock speakers in doors and back panels with JBL speakers, added two amps and may add another, there's four 9x6 JBL power series each in it's own box in the back seat, as well as two 12" kicker subs in one box in the back.. So the back seat is LOADED with speakers.. Problem is if I play it loud, it drains on my battery and I can see the battery meter on the drivers dash (beside speedometer and such), go down.. And the deck in the dash flickers and dims aswell when I play loud... What can I do to prevent this? Bigger alternator? Better battery? Something to do with more capacitors? Fuses? Etc? What can I do so my power in my truck doesn't dim and battery doesn't need to be constantly charged, also it would give my speakers more output if I fixed this problem