Without coming across as arrogant, you should honestly be able to look in the mirror and be able to say "I like me". Better still, you should love yourself. I'm of the opinion that if you can't like/love yourself then you can't possibly let other people like or love you. It's just self-acceptance. You need to like your flaws as well as the good bits! If there is a trait you really don't like about yourself then the chances are that you can change it and improve upon yourself.
I tend not to put myself down. I used to though, many years back. But I found that it gave other people the opportunity to join in. I can laugh at myself and don't mind being the butt of a joke, but I will not allow people to put me down. I find that most people will do, given the opportunity. So just don't give it to them! Be strong and people will soon be in awe of you. xx