Depending on the type of helmet you have, this might not work... my girlfriend has a Bern helmet, and that just seemed too nice to start slicing and drilling, but my Bell Faction model helmet was just dying for an upgrade.
This is a pretty simple project. All you need is:
About a foot or two of EL wire (I got mine at http://www.coolight.com/ )
An EL driver (also available at http://www.coolight.com/ )
1 SPST switch
Soldering Iron and Solder
Spray Paint (optional)
Heat shrink tubing (optional)
and a hot glue gun (clear scotch tape works too)
The EL wire I purchased came with instructions, but there are plenty of instructions available online if yours doesn't. Once you've got your EL wire working, take the red cable connected to the 9V battery and cut it so there is an equal amount on either end. Connect your switch and test it out.
Using a razor blade, I cut a section of foam out of my helmet right about behind where my right ear would be, making a space for the switch to go while also giving some extra room for a 9V battery. The battery and excess cable fit snug into the air vents in the back of this helmet on the right side.
At first I just used scotch tape to secure the EL wire to my helmet, but over time it started to fall off. Now I've used hot glue, and due to my sloppiness that may also start to fall off before too long, but you get the idea. If you pay attention to detail it can look really nice and people might even complement you on the street. Also a great project for a kids helmet, especially if they don't like to wear theirs.