Mic the acoustic guitar how you would normally mic it.
Plug the guitar's output into a DI and capture that signal as well.
Use a lavaliere microphone and place it inside the guitar in the middle of the sound hole. The trick is make sure it doesn't inhibit the artist's performance.
For the voice use a ribbon microphone or a dual condenser mic and place it in figure 8. Point the front of the mic straight at the vocalist and tilt it up a slight bit so the sides are more pointed at the guitar. The figure 8 pattern has great side information rejection and will prevent most bleed.
If you want a simple setup...
Use one condenser and position it in a way that it captures both guitar and voice in the amount and tone that you want. This ensures you have no phase issues or cancellation. It takes a bit of time moving around the mic, but the performance you capture will be exactly what the performance sounded like. This is my preferred way of tracking a singer/songwriter for this I prefer something like a Neumann U67 or a 47 if there's no U67 on location.
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