@Antti made great progress in back rack flexibility and stability in some months. Check in with him and his training log. Start with what is accessible, and as you gain flexibility you work toward where you want to be:
I like exercises that resemble the position
- PVC or Dowel Rod Dislocates(my number 1 exercise I do multiple times a day), if you can get to where you can do full grip straight arm, clean grip or closer, Dowel Rod Dislocates, that will solve most upper body restrictions
- Klokov Press
- behind the Neck Press
- bent arm bar
- side rack and side Press
- doorway pec Stretch
- back pull(one of my favorites)
- wippet and shoukder shimmy
http://yoga-horizons.com/pdfs/Fatmans-Guide-to-Cable-Training-2.pdf
I can't remember everything that I did or have done. Most of it is in the logs. But what comes to mind:
-doorway stretch, a lot of this
-rubber band shoulder dislocations
-massage, both with fingers and massage balls, pecs, biceps, shoulders
-press behind the neck, or should it be called the Klokov press? Isn't the difference just the grip width? In any case, I use a wide grip, it's about the widest grip I use on any lift. I think this exercise is #1 when it comes to improving my mobility. But I suppose one should be warned that many consider the lift dangerous. For what it's worth, since we're on this site, Pavel sees the PBN as a fine exercise if you don't have restrictions for it, and actually recommends it as an effective flexibility exercise for squat grip woes in Beyond Bodybuilding. (That's a good book.)
-dumbbell flyes, to stretch the chest
-lots of time under the bar - likely the most important thing in general
-improvement in technique, like bringing my elbows more up front and closer to the sides, and eliminating any weight from the hands - the bar is 100% on the back only
-I have grown to like wrist wraps
I think it's the best to start with a grip width that's comfortable or just on the edge of it. Like from the collars, if need be. Then narrow the grip a tiny bit during each session between sets and then between sessions, while improving your form.
And of course, when I got to use bigger weights, the bar started bending nicely and it helps too.
Back when I started, I had my wrists on the bar by the uprights. My original goal was to have all of my fingers on the bar, since that's in the IPF rules. Now I think I get to bench grip width. I wouldn't call myself that flexible. But I'm past my original goals. I could say that I'm still getting better, but I'm not sure I'd call it better since I'm not sure if the extra flexibility is of any use. But I'm fairly sure it's of no harm, and I'll continue experimenting with different widths in the future.
From the exercises I did, I only do the PBN anymore. I suppose I rest in the doorway stretch position every now and then, but I don't really practice the stretch at all. I massage when needed.