As mentioned in Ajahn Lee's method 2 below, you should not focus on the nose if you have nervous problems.
I would suggest whole body awareness with abdominal breath to maintain a centered but broad awareness
. I would also suggest listening to a lot of Thanissaro Bikhu's talks (he teaches ajahn lee mthod) at dhammatalks.org.
Basically, you start with body scans, just moving your attention to different parts of the body, and trying to feel the subtle energy there (Thanissaro refers to this as "breath energy" is his talks). Then after you do this for awhile, focus on the abdomen when breathing, but keep a part of your awareness on the whole body at the same time.
The window of your awareness will begin to shrink up after a while (you'll just be focusing on the abdomen instead of the whole body+abdomen). When you notice this go back to doing body scans to bring the whole body awareness back.
When the window of your awareness is large enough to encompass your whole body, the mind does not slip into the past or future...but when it shrinks up to just the nose or abdomen breathing it is easy for your thoughts to slip into the past or future.
Its easy to do body scans while at work, and maintain whole body awareness during the day even without the breath. http://www.accesstoinsight.org/lib/thai/lee/inmind.html