Damn, that's a mess. As mentioned, zip ties are helpful. Try to route cables in less obvious places and zip-tie cables together that run to close places. Just play around with it and see what you can come up with.
honestly your best bet is to unhook ALL of the cables and make a plan of attack. Try and use the fewest amount of cables possible and zip tie all the unused ones together and hide them somewhere. Run as much as you can behind the motherboard tray (if you have one) if not try and run them behind the hard drive and and CD/DVD drive cages.
honestly do what poker said, also take the back panel of the case and if you have USB 2.0 cables there should be little 1x1 inch wholes on the back panel, run it through there, also put most of the cables tucked away behind the bakc panel