I appreciate the tips, but please don't confuse ruby and rails.
I'm scared for the poor newbies who don't know where to look for help when things go wrong. If they think they have a ruby problem when it's actually a rails problem they'll be stuck looking in the wrong places for help.
A good one will give you exercises you can do to strengthen the muscles in your wrist. Mine gave me things I can do while I'm sitting in my chair thinking, or just on a quick break while still in the office.
Anyway, I agree with the parent: gadgets might help, but the real fix is likely exercise.
No matter how long you've been programming solo, you can't expect to sit down with a pair and have it "just work". Sure, it happens sometimes, but if it doesn't, you should practice it a bit before discounting the idea.
There are a few things the navigator can do to help out the driver while the driver is typing, but I hesitate to focus on them. In general, both programmers should be engaged. Learn to talk out ideas with your pair in short segments.
TDD helps. Test driving code naturally gives your pair many opportunities to inject their ideas without ruining your train of thought (ie: stop and discuss things at least after every test, preferably much more, but that comes with practice).
I think the problem with telling most people not to touch your screen is that often they don't see an alternative. So I always make sure I have a pen with me. If someone touches my screen, I usually just hand them my pen and say "Here, point with this".
Not only do you avoid the finger-oil issues, but it's also more accurate.
I love to tell people that the hardest parts about writing software have nothing to do with code. It all has to do with communication.