OBE events and "expectation"
The concept of "expectation" is very important to your ability to have an OBE or lucid dream. In short, you must tell yourself it's perfectly normal to have an OBE, millions of people have had them before you, and millions will have them in the future.
Beyond that, it's important to say -- and believe -- that you've done everything you can to set up the conditions to have an OBE tonight. Maybe you've practiced yoga and moved your awareness around your body, you've relaxed your body, maybe you've listened to TMI recordings, or practiced the techniques discussed in Astral Dynamics. You've also conserved your energy (like Don Juan in the Carlos Castaneda series), you've stayed away from sex, alcohol, drugs, caffeine, sugar, etc.
If you've done all these things, then you can confidently say to yourself, "I expect to have an OBE tonight, and in fact, I'll be surprised if I don't have one".
A "cool down" and setup ritual
When you go to bed, give yourself a little time for a ritual. First, take the time to relax, and tell yourself you deserve quiet time, time to have an OBE. I often repeat these words as I go to bed:
There's nothing to do, there's nowhere to be, I don't need anyone else, and nobody needs me.
I actually have a much longer statement I use most nights, but if I'm particularly tired, I just repeat this phrase as many times as needed until my mind is clear. If you prefer chanting, that's certainly another approach. Some Zen sutras are great for chanting slowly at night to clear the mind, and I believe that's exactly what they're for.
As a final part of your nightly ritual, make sure you have an audio recording device, perhaps a dedicated recorder, or a voice recording application an a smartphone. Set the recorder beside the bed, and then think (or say) that you expect to use this tonight; you expect to have an OBE (or lucid dream, if you prefer), and when you do, you'll wake yourself up, reach over, pick up the recorder, and make some notes about what just happened. And then if things go really well, you'll have a second OBE, and possibly a third.