One thing that we did was use some of the interface cables from the NXT to RCX cable. You can take two of these cables and combine them at the RCX connection and make a very long cable. If the cable is longer than that that you need, you might want to move the NXT closer to the motor in question.
You can get the cables
here
Also, HiTechnic sells "Extra long cables",
You can find the at the bottom of this page
I hope this helps out.
Regards,
~FTC #1, Team Unlimited