No problem 

I would like it if you would take a moment and consider yourself in these two scenarios:
A friend approaches you asking for a “favor”.  Initially, your instincts kick in and make assumptions to help determine your answer: who is it that is asking?; what sort of favor could this be?; I’ve seen this coming, how much can I afford to lend?; is this a true crisis or just another case of him/her crying wolf?; etc., and is usually based on the answer to the very first point.  Most of the time, if they are a true friend, any assistance that you can provide, you will, with limited expectations.  Now, when the favor has been done, and the requester thanks you, what is your response?
No problem
You’re welcome
Don’t even think about it
Just pay me back when you can
A colleague asks you to do something for them, and it is outside of your normal scope of work.  When they thank you, how do you respond?
You’re welcome
No problem
A superior asks you to do something for them, and it is outside of your normal scope of work.  When they thank you, how do you respond?
You’re welcome
No problem
The point I am trying to make is that I want you to give serious thought into what your automatic responses are to these requests. Consider each person making the requests, the amount of time it took you to complete the task, the affect it had on your own personal obligations at the time, etc. Now please consider your responses again, and decide if that was the right one.
I’ve noticed that, when I respond with “You’re welcome.” I feel a stronger sense of respect for myself, especially in a professional environment.  When it’s a good friend, there is never a question of a “No problem, anytime!” response because I know that I will get the same in return when it’s my time to ask.
The next time you are at work, and a co-worker or supervisor asks you to do something, really think about the task.  Consider how long it’s going to take, if it is a part of your job description, if it will affect your current schedule and the adjustment you will need to make, and if it will benefit you in any way.  If you feel the request is acceptable, be aware of how you respond when they thank you for what you’ve done.
I’ve found that saying “You’re welcome.” feels more accomplished, I feel less like I am being taken advantage of because I am willing to do so much, and I am getting the appreciation I know I deserve.