Just three main points after i mention your strap.
This is easy to fix by the way, but your strap does not sit correct, the widest part of the strap should be on the shoulder.....i cannot see you strap clearly but it looks like most of the width is on your back.
A good strap should be at least 4"/100mm across the shoulder, any padding included. So have a look at that and more on strap position later.
You have a lot of tension in your playing is the first point.
Try some stretches and warm ups before you play, http://tranquilitycs.wordpress.com/2...in-and-stress/
Next point is a personal one, but your strap is on your neck.
If you feel your neck you will feel ot is soft tissue, but of you move your hand along the shoulder towards the edge the tissue becomes more solid.
Try and get your strap a bit more to the 'outside and use this solid part of the shoulder rather than the neck.
It may take getting some used to, but if your shoulder line is stable the strap stays stable..if the strap slips off, then you now have an indication that your shoulder line has changed....its a good indicator that you have changed posture for some reason.
Final point, try not to look at your fretting hand, I know it is hard not to, but there is no need to.
What you have is a habit, because you spend as much time looking away as you do looking at your fretboard.
Notice how you look.
Your shoulders are forward, giving them the rounded shaped, you drop your head, then turn it.
So you add tension by stretching the neck when you drop it, then add more tension by turning it to look at your hand.
This can lead to the rotor -cuff area working harder than it should, ot is trying to pull back your shoulder line and support it from the back.
Your Pectoral and Lateral muscle groups are not involved in the support the way the should be. When your posture is correct your shoulder line is straighter and balanced (now dipping, raising or favouring one side).
The exercises in the link i posted will help with this.
If you have to look, then just turn your head and look down with your eyes, try not to dip the head forward and turn.
Stand in front of a mirror and learn to turn your head while maintaining an even shoulder line.
There is not need to add any other movement, as i mentioned with the strap position it will start to slip off if you do drop your shoulders.
I deal with a few 'readers' , players who sight read for a living, who have the habit of craning their necks when reading parts.
The habit is they stretch the neck toward the music stand, the correct position is to bring the stand closer to you, so now the need to crane the neck forward has been eliminated, but the habit may remain, so playing in front of the mirror helps a players see the fault, but more importantly, helps the feel it.
Sometimes they need glasses, or have a prescription up-dated if they already wear them...such issues creep in over time so tend to feel comfortable until the problem occurs.
Posture related issues are usually gradual ones over time, the moving further away each time until the body can no longer support the use.
Since we cannot have a mirror when we play live, the stretches and exercises help us feel what correct feels like, so we do feel un-comfortable when we stray from it, as does the strap slipping.
I would say correct the shoulder line, add some stretches and warm ups and the head turning issue will take care of itself, of the strap is an issue buy a better one, it is one of the things you can buy to make life easier.
If you have any further questions i will be happy to help if i can.