The stretches at the front of Mastering the Rubber Guard are really good. I must admit I don't do them religiously every day but I have noticed a difference in my flexibility since starting to do them on a regular basis.
The best stretches I have found for Rubber Guard are all from using the RG. The standard stretches got me pretty far, but setting up the positions on a compliant training partner (especially double bagger) and then holding the clinch while my training partner works to get free for 30 second rounds has really freed up my mobility.
I've been using a book called Stretch Therapy by Emily Francis as well as Yoga for Martial Arts. Guys at the school make fun of yoga, but since I've sstarted using them my flexibility has really improved.