The best way to hide a double-lift is to perfect the technique. With a perfected technique, you are much less likely to run the risk or revealing how you did it.
Double-lift at angles beneficial to your performance. This can be practiced in front of the mirror and with your family.
Positioning yourself at the proper angles to your spectator (and they all vary depending on who you're performing for and where you are performing) will prevent them from catching on.
Talk to your spectator. By misdirecting him with conversation, he will focus on you, not the cards.
I get asked all the time how magicians are able to pull off such feats when they are right in front of us - and the answer I always say is that they are talking to you as they do it.
Actually think and feel that you are presenting only one card. Knowing you are showing two will subconsciously allow leeway for messing up, which you cannot afford.
Study your actions when turning over just one card. Watch yourself turn over just one card in front of the mirror and memorize these actions.
Copy the actions from Step #5 to your double-lift. If you present the cards the same exact way as with just one card, how will anyone know it is not really one card?
Know your spectator. Do not try and do crazy double-lift tricks for someone hellbent on heckling or revealing your secrets. These types of people are usually unable to be misdirected.
Do not try and win a battle. Walk away graciously and blow someone else's mind.