This was probably the first move i really got stuck with. I practiced it for 3 months, every now and then, and still ppl caught me "Hey,I saw two cards there...". So I practiced alot more before showing it again. I worked with it for about 7 months further before it finally became like a second nature to me and I got the confidence to perform it. In January I performed it at least to 50 ppl, personally in close-up, none of them catching me or even reacting weird when I did the dl.
My hints are:
-First of all, make sure you are holding the deck right in your left hand! This was my biggest mistake. I held it too high. Watch videos and clips to see how the mechanics grip exactly is. IT IS NOT THE GRIP YOU ARE DEALING CARDS FROM, you hold them more firmly in the mechanics grip. The left first finger should be able to curle a little above the outer end of the deck.
-Practice different methods, I mostly use the hit lift, but if the cards are in bad condition (borrowed deck) I always use a get-ready before the lift (pinky break under top 2cards). You'll find ur own at the end (Yes, it will take time. But remember, the time spent on practicing dl's that u won't use is not a waste of time, it trains ur hands to get use to the cards)
-Have a deck and do dl's everytime u watch TV, after sometime u don't have to concentrate on the dl and can actually follow the program.
-Practice infront of the mirror. Probably the best way to practice. NOTE: Be careful that u don't close ur eyes just as you are turning the double. This might sometimes happen as a reflex as you are aiming to make the dl unnoticeable. Sounds silly but it's true!
-When performing at first, use cards with a white border (for ex. Bicycle, not Bee's) as the doubles will be harder to spot that way when turned face-up on the deck.
-Get the turnover part right! It might well be the most important phase of the dl. It should be almost like turning pages in a book (after all, u are just dealing with paper!). It has to be so natural that some spectators even watch away from the cards when doing it (they think nothing could possible have happened there).
-When you do the turn-over you can do an "adding convincer", by bending the cards so it comes a knacking sound. Because of some reason this fools the spectator to think there's just one card.
-If you use a get-ready. Always have a short break between doing the "get-ready" and turning the double face up. Make the get-ready->talk to spectator for like 5 seconds this way he won't think you just did something strange to the cards-->then turn the double face up. ( Here I sometimes even tell him to watch carefully at the cards, this might sometimes make him watch less carefully at the cards thinking I'm doing something else

, fool ) NOTE: It's good to keep on talking WHILE turning the double so it "flows in". If u stop talking right when turn over the dl, the spectators thoughts are more like "ok here it comes...I'll catch him" rather then "okey, so there's the 2 of spades"
-Never do the DL too slowly or too fast. Doing it too slowly might separate the cards, do it ultra fast and ur caught right away! Again, do it like turning over a page in book.
-Don't be afraid to aligne the two cards with ur left first finger at the outer end when u turn it face up, do the same when you turn them face down. At first even this might look suspicious but do it smoothly and naturally and no-one will notice.
-If all the books and hints haven't helped, buy Gregory Wilson's "Double Take"-DVD. I have it and it's great, it was worth the money. You need to get this move correctly, it is that effective!
-Last but definatly not least what all have been told you: PRACTICE, PRACTICE AND PRACTICE. And when u got it down, practice some more. The dl is a matter of skill, not luck!
If u got any further questions about the dl, pm me and I'll be happy to help you (if I'm able to) or show u a video clip of me doing the dl or any other move I just mentioned...