Take them in overcast natural light, avoid sunlight and flash as you get a nasty glare on them. If I were you I'd pan back from the pictures and do distance shots, I personally find them more appealing then close ups, and also you can't get a good idea of how the item will look in full.
The sofa picture in this listing is good:
http://www.etsy.com/listing/86751134/large-guitar-original-print-based-on-aJust open up the curtains and take it in natural light. If you don't have a bright/naturally lit room in the house, borrow a friends! Pan out more, maybe have the guitar on the seat of the sofa with the frame above. Play around and think of composition : )