The best thing I learned from my non-lucid dreams is that doors are awesome. What's behind them is always what you expect to be behind them.

I once had a non-lucid nightmare where I was in a house and just knew there was someone else and that this person was evil. Just before a door opened I got lucid and took the opportunity, called out the name of a friend of mine and asked if it was her while the door was opening. That's when the nightmare turned into a lucid, uhm, love dream

Doors are awesome in general. If I expect some special person to wait behind the door, that person is there. If I expect to enter Mordor through a door, there will be Mordor behind that door. It only fails if I want something to be there, but am too insecure and not confident enough. Wanting something to be there isn't enough, I need to really expect it to happen.
This is why I love hallways with a lot of doors. If one door doesn't keep at hand what I want it to (because I was too insecure and only wanted, but didn't expect it) I can just go to the next one.