The secret (this is going to be a let-down but it is the truth) is working hard at the barre!  I was a great athlete in college.  I was fifth in the Olympic Trials in 1972 as a pole vaulter.  Then I took up ballet and I could not do a double tour to save my life for three years.  The reason is because I kept trying to muscle it - to jump higher and spin faster.  But all that was futile because my body alignment wasn't good enough.  I was not rotating around a good center so I spun out of control.

Work very hard on a strong straight torso at the barre while you do your romb-de-jambs, developes, and adagio work.  Never compromise the lower back, shoulders and neck.  After years of that - you are ready.

When you do your double, start with a pique onto a high releve just for a moment, then drop into a good demi - but do not try too hard!  This is very important.  Get into your mind that this is easy because if you try too hard you will fling your lead arm out too hard and you will twist your torso too hard - then you are dead before you even leave the ground.  

Its OK to cheat a little by starting 20 degrees into the first turn.  First start the head turning towards the first spot a bit earlier than you think.  Let the lead arm open a bit earlier than you feel is right, close the following arm early as well - and don't try to snap it in hard - this will through you off.  Just close the following arm.  The only thing that should be sharp and hard is your spot.  Make it very sharp!  Hold you body vertically straight!  That way you are in total control - you may land after only 1 1/2 rotations but you will complete the double on the ground and most people won't notice.  But you cannot do that if you land out of control.

Finally when you land do it with a peaceful - not a triumphant demeanor - you want to make this look easier than it was.

Best wishes for the holidays.  Happy doubles.