This mod adds a correctly sized antenna, effectively increasing applied power and thus allowing operation from much greater distances. I have seen increases of tenfold in control distances after this mod.
This mod will show how to open the transmitter (RF-04/RF604), create the antenna wire, locate the place to attach the antenna, mod the case, and the final construction. Enjoy.
Note: this instructable was derived from material presented in the Strobist blog, here, with the actual article here. The original description is here.
I found this model on ebay, from a great supplier: Magic_Trigger. There are others on ebay and other locations, but I can't speak to the exact design they use or how to modify them, but similar techniques can be used.
Step 1: Open the Transmitter
Start by opening the transmitter body: remove the single screw at the front of the bottom of the case. It's a phillips head and small. Once this screw is removed, pull the top off at the front. Don't worry about loose pieces, because there aren't any (except for the top and the screw).