What are some good ways to improve one's self esteem?

My self esteem is OK but I’ve always felt very insecure about myself, especially in the company of others and especially amongst new people. It’s got better as I got older, but I still feel very insecure often and tend to think negatively about myself.
This seems to affect my friendships and relationships, as I believe I’m not worthy someone else’s liking. My life is great otherwise and I do have many friends and a great family. This just seems to be something that is happening inside my mind.
So can you give me some encouraging advice on how to boost my confidence?