I usually scan my paintings: Wait until the painting is dry, varnish it, and scan it in pieces on a flatbed scanner (any size), make sure the scanner makes no color adjustments to each individual piece, then stitch it together in photoshop with file>automate>photomerge. Once it's stitched together you can make any color corrections necessary, you may need to use the rubber stamp tool to remove dust and glare if you have high relief brushwork.

Recently I've started getting good results taking photos: You'll need a nice camera like a Canon Rebel or better, a tripod, a polarized filter for your camera lens and polarized lighting gels (reduces glare). I got this whole set not too long ago for about $1000.
There are plenty of good tutorials online for how to set up the camera and lights, beyond that I would just recommend that you bracket your shutter speed (take multiple images at different shutter speeds) and use photoshop to file>automate>Merge_to_HDR to make your final image. That will give you better definition in the lights and darks.