This will be on a longish side, since it's a book, but first you need to understand why the mind is racing and why it's undesirable.

http://en.wikipedia.org/wiki/The_Power_of_Now

Start by relaxing instead of napping. Once you get the hang of that napping comes next.

25 minutes of laying still in dark and quiet room also gives a lot of benefits. I rarely sleep during my "naps" but still find it very relaxing and refreshing.

So, you might start by doing that.

They did a pretty interesting AMA on napping and sleep - probably answered there: http://www.reddit.com/r/IAmA/comments/1vyuz2/iama_scientist_...

Beer to calm the mind. Reduces working set. Encourages focus too. If done sparingly, can temporarily relief mold stress also.