I found an Ab Roller the other day (the one that has two handles and a wheel in the middle). I tried it out for shits and giggles. I did get a tremendous feeling in my abs that felt goooood. However, now my shoulders are killing me. Anyone ever use this tool? If so how do you keep your shoulders out of the work?
I believe you're referring to an Ab Wheel. The Ab Roller is a different type of apparatus. Along with a few other exercises the ab wheel is a staple in my ab routine. As well as working the entire abs it also puts stress on your arms, forearms, back and hip flexors. That's a good thing so consider using it on a continuous basis.
A standard barbell can be used as an ab wheel, also.
To protect your shoulders, you'll just have to be careful. As far as I know, there's not a way to do the exercise right and restrict the stress on the shoulders. You can do the roll-outs from your knees, and you can use a wall in front of you to limit your range of motion, but a full roll-out will definitely put stress on the shoulders, not that there's anything wrong with that.
If your wheel is set up to attach to your feet (will have straps) you can stay in the pushup position and use it that way, though it's a totally different exercise.
For more info on "The Wheel", you might check out the work of Ross Enamait. He has one video where he does full wheel rollouts with multiple weight vests on. I don't know how he survived.
I generally do barbell rollouts instead, as you can add more weight. This makes it much more difficult as inertia comes into effect.
To put less stress on the shoulders keep your elbows bent and lower than the bar. I don't know if this is possible with an ab wheel, due to the small diameter wheel.