Second, you should consider how the game might change over the next two rolls. If after your rool and that of your opponent you think he will still have a take, then it is normally correct to wait. If, however, there is a very good chance that he would have to drop, then again you should consider doubling.
Third, never forget you are playing another human being. How will your opponent react to a double? Does he take any double offered? Or does he drop lots of doubles quickly? You must adapt your cube-handling depending upon who you are playing. Knowing your opponent well is a key element to good doubling.
Finally, there is the position itself. The three elements I normally consider are the race, the structure and threats. If you are better in two out three of these you should normally be considering doubling. Look at the position above. Black opened with a 31 making his 5-point. White rolled 63 and played 24/15 and then Black rolled 43 and played 13/10*, 13/9. White rolled 65 and stayed on the bar. Black now has a very good early-game double. He is ahead in the race, has the better structure and some very real threats. In fact despite this being only the third move of a game White can only just take this double.