Help enough to build that reputation for being a team player. When you can't fully help, don't just say no, give them some starting pointers and offer to review, or assist more if they get stuck, so even then you are still being supportive.
Help people to help themselves. Don't just do something for them, guide them in how to do it themselves so next time they don't even have to disturb you.
Listen out for opportunities to use your skills to help people when they don't even realise they need help. I've lost count of the number off little scripts, tools or reports that I've knocked out in a couple of hours just because someone absent mindly moaned about something they found annoying. These things build your reputation far more than just plain helping, and if you're observant they are often simple and quick to do.
But also, make time to progress your big goals too, which does mean you have to say no sometimes.
Get help in return from those you have helped to move your goals forward faster than you can on your own.