My favorite technique for white backgrounds is simply to draw around the object with your pen tool and delete the outer portion to white.
If you don't trust your path selection skills, then put the guitar on a black background, use the "magic wand" to select the black background and either fill with white or delete it.
Either way, you're basically using a variation on hollywood's "green screen" technique. Works great!
Most of my photos are a variation on that with slightly more finesse. The problem with doing that is that reflections and highlights remain which is why I moved to the white backdrop.
Here's an example of what I used to do.
Take the pic on a couple of white foam boards:
I always shoot in raw which allows me exposure latitude not possible with jpegs. I set the exposure of the guitar the way I want then adjust the histogram to blow out the white background. Note that this step is not so easy on a white guitar because it blows out the entire scene. That's where LA's trick of selection kicks in but if it's even a little bit off it will look like a bad photoshop.
Finally, I crop out everything that shouldn't be there and photoshop away the creases and splits from where I had to use multiple foam boards. Note that all of this is done with the flash on-camera which has its own limitations, but with a white ceiling in the room you can get some really good exposures if you can diffuse that flash and angle it up instead of straight-on. This is the final pic:
With a real white background I no longer have to edit out the splits (yay!) and I get a more even light.
Issues with these pics: Look at the waist and the cutaway on the guitar - that white mottled pattern is a reflection from the foam board that's been exposed away, but the reflection remains (look at the first three pics again and you'll see the source and then they're gone). Try to fix that and it will look terrible. No one probably noticed until I pointed it out, though.
Here's another problem that I have in a lot of my shots, especially of arch-tops:
Look at the top of the pic and you can see a complete reflection of the room I'm in. Lame!
Here's another.
Look at the bottom edge near the knobs. See that dark shape on the side of the guitar? That's the floor where the white foam board wasn't big enough. Lame!
All of these nuisance reflections are (for the most part) resolved with the continuous white background. Every one of these photos was taken with a very simple rig, though to be fair it's a bit pricey.
Canon 5D Mark III (doesn't matter - a Rebel would do just as well)
Canon 580EX flash with diffuser (matters a little - just need lots of indirect light)
Canon 24-70 f/2.8L lens - matters a little because lens has nice contrast - aperture is almost always f/8 or above.
The trick with the single flash is to diffuse it and avoid specular reflections which is most of the hard part. You'll notice that I rarely ever shoot the guitar straight on - that's to avoid direct reflections. I recently installed LED recessed lighting in my home office and I can actually shoot with just them on high, but they cause reflections.
And now you know all my secrets.