The key is to find something to focus on that can maintain your awareness without causing too much arousal that you stay awake.

Sometimes I do a mix between FILD and DEILD. In the brief waking period, I choose some tactile sensation to focus on. It can be from the previous dream (i.e. the feeling of my weight and movement while walking in a field) or a generic one like rubbing my hands together. It just has to be something simple that you can vividly imagine. That is all you focus on; if you get too caught up in HI or vibrations, you might not fall back asleep. Eventually a dream scene will form around me and I will actually be feeling the imagined sensations. That is the signal that I am in a dream.