In this economy, you may be working with a smaller office staff which means more assigned duties and many unassigned duties brought on by you, the "answer" person.
If everyone in the office seems to comes to you for answers, it does not necessarily mean that you are being used or dumped on. Instead it may be that you have made yourself the office expert (on the surface, not a bad thing.) So it is logical that you are the person co-workers and managers seek out when they need help.
Being seen as the office problem-solver can be a good thing, but it can also become a burden, especially if you have difficulty admitting that you do not know something. When you pretend to know what you don't know, you have to work harder to get answers or find a solution. So what can you do? You can start by admitting that you do not know the answer. Say simply, "I don't know." You may offer a bit of 'where to look' guide dance, then move on to what you were doing. When you admit that you do not know, a co-worker may offer helpful information.
Remember, you do not have to have all of the answers. No one will think less of you, unless you have gone out of your way to make yourself known as the office know-it-all.
Learn how to pass it on:
1. If you find managers are bypassing their own staff to ask you for help, etc. be respectful but give them an answer that will send them back to their staff person "Oh yes, I will tell John where he can find that information so he can pull it for you."
2. When a manager tries to leave work with you to be copied because Mary's are not as sharp as those you make. Say you are busy right now, but ask that the copy be left with Mary, explain that you will stop by Mary's desk and show her how to make those sharp copies. Be careful here you do not want to alienate Mary or make her think you are telling her what to do. Be as honest as you can, tell Mary that her boss wanted to know how you made your copies so sharp, then let her know your secret.
3. Learning to say no may not an easy thing to do, but you must learn to speak up for yourself. Know that you do not always have to know or have the answer.