That used to happen to me a lot, but I find that if you start the descent slight early you rarely get the DRAG REQUIRED message. About 5 miles before predicted TOD go to DESCENT page, push DESCEND NOW and EXEC. It will descent at about 1000fpm until it reaches the calculated path when it will go into IDLE and fly down the slope. It seems that if it intercepts the path in level flight it will overfly it a little which eventually cases the drag required situation. However, as was mentioned before ATC can spoil your day and unpredicted winds can but the winds seem to be much less of a problem doing it this way. And yes, keep the descent calculations going in your head at the same time, it helps a lot.
Hope that helps.
Lazlo