As the others have said, the goal of the technique is to get you back into a dream. You have to be asleep to dream, and if you can't get yourself into the dream then you obviously can't exit your body.

A way of doing DEILD that I like the sound of is imagining the scenario you want to be in. This gives you more control, because it means that your body will already be "out of bed" and in the action of wherever you want to be. It also means you will definitely know when you are in the dream; it will be quite clear to you.