1. First 6 months is the honeymoon
Sure things may be going great in the beginning because the other person wants to make a great first impression. But after that- true colors tend to come out. Don't commit until you know the person in all sorts of situations... It just makes sense.
2. Rose colored glasses
People in love tend to not see a lot of things that may be obvious to others. Try not to close your eyes to things that may be red flags. This is why keeping your options open and reminding yourself of how a relationship should be, is the way to go.
3. People want what they can't have
If you create the illusion that you aren't putting all your time, effort and energy into a single person, unless they are worth it- this will create a sense of value and respect for yourself. Of course people don't want something that comes easy to them. Do the opposite.