I agree fully with fOrceez. Repetition is key. Way too many people fail at lucid dreaming simply because they try MILD one week, have no success and next week they try to focus on WILD. You need to train your brain over a long period of time (it varies from person to person) in order to become fluently successful with a technique such as DILD. Eventually you will add your own little tweaks through experience and perfect a DILD technique that works just for you.

And yes, focusing too much can be just as damaging as not trying hard enough. You don't necessarily need to repeat it till you fall asleep. More passively suggest to your mind that you are going to have a DILD tonight and that you are going to recognize your dream signs.