I have been successful in stretching my handspan quite significantly. When I started playing exactly 12 months ago I could play a 9th with a stretch. Now I can play a tenth (C-E) comfortably with both hands. Here's what I did:
Static Passive Stretch:
Place the hand out stetched with the thumb and pinky against a surface (like the edge of a table, or the keys of your piano). Gently push, forcing the angle between 1 and 5 to widen. Be sure to have the wrist so that the thumb nail is pointing towards you (not up) - you should feel a stretch in the palm muscles, not the joint! Also, be sure never only to increase the range of motion until you feel a good stretch and NEVER until it hurts. Perform the stretch for 30 to 120 seconds. As the palm losens, increase the stretch.
Hopefully, you will find that after this stretch you will be able to access your current greatest range of motion.
Dynamic Stretches:
Here I use some simply dynamic exercises to enable me to hit longer intevals during place. Try this:
Bounce (slowly to start with) these intervals C - C, C - D, C - E, C - D, C - C...
You can adapt this to fit your hand better. For example, some people might want to go further / less depending on their physiology. As your dynamic flexibility improves, you can increase the speed of this exercise.
Practice Rolled Chords:
Next I like to practice rolling very wide chords - 11th, 12th, 13th etc. I take chords from pieces, such as Chopin Etude 10 - 11 (which I cant play at all btw!). But I think it is a good dynamic exercise to practice this as its good for the wrist as well as the hand. Plus it works great to losen the hand after the previous stretches.
Practice Chords:
Finally, I like to place random 1 - 5 - 10 or 1 - 6 - 10 chords, using the pedal to connect. The chords arent really that important (to me at least), but I try to come up with little improved progressions that sound nice. At the moment Im only doing this on the white keys. The purpose is only to practice hitting these chords in performance.
I have been doing this now for 6 months or more. My reach has increased notably, as has my ability to pull of wide chords in pieces. I have not suffered any pain or stiffness at all. The only thing I did have was an odd swelling on my LH ring finger, but that turned out to be from the cold (I had been working outside for several days previous!) and nothing to do with playing the piano.
Hope this helps!