Calm down! Your audience will not know you messed up unless you let them know you messed up.
Decide if you want to start over or move onto the next trick.
If you decide to start over, cover it by saying the cards are not shuffled, or mixed well enough.
If you decide to move on, be sure to do so quickly. Keep in tune with Step #1. Remember, nobody knows anything unless you make it known.
Stay with the same spectators. Do not walk away if you mess up. Since they do not know you messed up, it will seem as if you are being mean and/or teasing them, which comes off as amateurish.
Shift your approach in double-lifting to one that is easier. I wrote an article on how to double-lift. Try the techniques in there if yours do not work.
Take a deep breath and act as if everything is going according to plan. Performing magic involves a great deal of acting. If you look nervous, the spectators will get nervous, and their enjoyment level will significantly decrease.