40 minutes is the minimum connection time for American at DFW. I connect through there frequently and have only a 40 minute layover tomorrow - I'm not worried about it at all.

The night before your flight, take a look at the planned gates and terminals and match it up with the Skylink map. There are 2 entrances to Skylink per terminal, and you can waste a few minutes if you go to the wrong one.

For example if you land at A23 and are flying out of C27, take the Skylink at A29. If you walk to the one at A16, you'll ride the Skylink past A23, to the stop at A29 and then onwards to Terminal C.

If you are connecting between a high gate at Terminal A and a low gate at Terminal C, the walkway / corridor will be faster than Skylink.

I can't find a map that features both Skylink and gate information, so mash up these two:
http://www.aa.com/content/images/abo...rminal_DFW.gif
http://www.dfwairport.com/cs/groups/.../p1_007099.gif