Schooner and cjd basically said what I was going to; make sure that the area that is reflected in the guitar is a dark wall or sheet and angle the guitar so there are no 'flares' of light.
For example, if my tripod is set up in the center of a room, to the right of the tripod is a hung sheet (over chairs or ladders) parallel to the focal plane.
The guitar is directly in front of the camera, about 10 feet, and angled toward the sheet on my right.
Zoom in (or use 100mm) lens, which is what portrait photographers do. If you use a short (35-50mm) lens and just get physically close the proportions, edges and vanishing point become progressively fish-eyed the closer you get. So, back up.
I try to avoid flash, and use indirect flood lighting that is placed on the floor and bounces off the ceiling and/or wall, and then a small, low wattage spot (think small halogen desk lamp, at a distance of 8-10 feet from subject) to add highlights.
I'm not at the computer that has my pictures on them, so I can't post an example of the above setup, but trust me, it works.
Oh, and even with a tripod, I like to set the 2 or 10 second timer, so that I'm not touching the camera at all during exposure. Even the slightest jiggle can change a sharp picture into an okay picture - it won't necessarily look obviously out of focus, but it won't look as interesting or vivid if there is even the slightest blur.
-K