I've been playing guitar for 14 years lady and lass, and being rather developed in the chest department, learnt at a young age to deal with the prob when sitting down - and it explains my posture.

When I sit down and play guitar, I naturally hold it slightly outwards at the bottom, and slouch down with my chest squashed and under it, avoiding the underwire bite. My chin almost rests on top of the guitar body, and I have found the wider the guitar, the more easier to rest the cleavage part of my chest onto the back of the guitar.

This does tend to give a strange angle to the posture though!

The other thing I do, is hold the guitar at a slight angle from the right, and sticking further out at the left.

This is a hard question the more I think about it. I guess my breasts have got a bit more squishy over the years and I've not noticed. Also as my chest grew, I automatically adapted.

Very rarely do I put a beer shelf on top of the guitar - although I reckon it would be a good lifting exercise.