Guide to reducing Interference

First Read the Standard Radio Page here
http://www.robowars.org/wikka/StandardRadio∞

Then, for some specific technical advice, have a look at these pages

http://www.technobots.co.uk/technical/guidance/EMI%20and%20how%20to%20deal%20with%20it.pdf∞
http://homepages.which.net/~paul.hills/Radio/Radio.html∞