Know your limits when getting work; Don’t over commit to an already full schedule; Take work as needed not as you feel obligated.
Know what your limits are, its good to have stretch goals and things to reach for. Just don’t put yourself in the position of constantly failing because you over reach all the time. You should keep very good track of your schedule and know where you are supposed to be at any given time so you don’t end up in the position of having to be in two places at once. Unless you are Doug Kinney its going to be pretty tough to pull off.
So get a planner and keep it on you. I don’t suggest a phone, while its a good thing to have and sure you might as well duplicate it, having an actual planner is much better for me. Its quick access unlike your phone, easier to update and has the added benefit of not making it seem like you are doing anything other than checking your schedule for the person you are speaking with. You wouldn’t want them to think you were playing video games or texting would you.
If you do by some wacky coincidence end up over extended then be honest with the people involved don’t make up some crazy story about aliens and Chiclets and a monkey on a rampage. If your honest about what happened and willing to make some compromises then not only will the person who is being let down not feel taken advantage of but they will also be more likely to work with you again because of how professionally you dealt with the situation.
If you keep track of what your doing, stay organized and on top of your schedule you will at some point still run into a conflict. Just remember to be a professional about it and you will be fine.