Something is making you give in...and as long as you keep doing that they will keep asking. You just have to stop doing it. It's not easy to say no and follow through but if people are taking advantage of your good nature - they don't really value you.
Don't promise to do something, don't tell them you'll think about it, don't give them any reason to think you will do as they ask.
Eventually they will respect your boundaries - as long as you keep them set.