It's about setting boundaries.
Firstly, what things are you happy to do for people as favours (ie you won't feel resentful when doing it) whenever they ask? Secondly what things are you happy to do occasionally, but not as much as they are asking for, or you want a bit more give and take? Lastly, what are the things that just flat out annoy you every time?
Once you are clear on what things you will/won't do it's much easier to maintain these boundaries and be assertive about them. Say yes to stuff in the first category and no to stuff in the last. Most stuff that you're struggling with probably hits the second category.
This is harder to deal with, but for each situation that comes up work out what you are happy to give and stick to it. eg giving someone a lift once a week is ok, every day is too much. Then next time they ask, state what you can give them something like:
"I'm very busy at the moment - I can do Wednesday if that works for you".
Specify what you are willing to do, and it's up to them to either accept or not. You aren't saying no, just setting limits that you are happy with, on your yes's.
If someone suprises you with a request and you're not sure - give yourself breathing space and claim "oh let me just check with DH/the calendar etc, I'm not sure when I'm free, I'll let you know" then figure out what you are comfortable with before replying.