My coworker is having marriage problems and always asks me for advice. How do I tell her I don't want to get involved without being rude?
You're right to resist stepping into this particular vat of quicksand. If you've tried the basics (avoiding eye contact, continuing to type as she "shares"), try the self-effacing angle, i.e. "Gee, I don't know why you ask me for advice…I just do what Oprah tells me!" If she still doesn't take the bait it's time to be super direct. Say, "I'm sorry, but I'm just not comfortable giving you advice. Marriage problems can be so serious and complex that I'd never forgive myself if I said the wrong thing." Then (even if you're at your own desk) casually get up and walk away so she can process what you said.