I love my job 99.9% of the time (which is rare, I know...) and it makes me not want to rock the boat, but I have a coworker who constantly delegates things to me that she should be doing. We are peers! How can I gracefully decline without looking incompetent or petty?
It is hardly petty to set boundaries, especially in the workplace! And as for looking incompetent, it sounds like in this case, your competence has set you up for some extra work.
While I'm all for the notion of being a team player and helping out every once in a while, you need to make sure you aren't taking on too much outside your scope or your work will suffer.
I'm a huge fan of honesty and feel with a little finesse you can tactfully deflect with, "I'm flattered you consistently come to me for help, but I'm a bit stacked at the moment, so I'm afraid you'll have to tackle this on your own this time around."
Nice, but assertive. And it lets your colleague know you are onto her and that the work you've done in the past is a favor vs. an addition to your own job description.
Take that time you earned and focus on your own career. Sounds like you are going places!