If you're looking for a simple way to add a battery to your LilyPad project, the LilyPad Coin Cell Battery Holder does the trick. It's a simple board which holds a 20mm coin cell battery (like a CR2032) and gives you 4 connection points (two positive and two negative) for sewing into your project.
The LilyPad Coin Cell Battery Holder is a design conceived by Felicia Williams of Boulder, Colorado.