Do you know where the issues come from? Did you grow up with someone telling you that you weren't good enough? It helps to figure out where it comes from and then reason out why that person was wrong in telling you that you aren't good enough.
Then, like the others have said, you have to start building on the positive qualities, and it is like exercise....you have to do it every day. If you don't do it often enough, it won't stick.
So every day, write down your positive qualities, and read them. Out loud. Look in the mirror every morning and tell yourself that you are beautiful, lovable, and deserve happiness.
It may feel awkward for a while, and it will take time (no quick fixes for the deep stuff), but after a while, you WILL start to truly believe it.
ETA: Oh, and for me, living my life with integrity makes a HUGE difference in how I feel about myself. I try to be honest always. I try to be up front about things. I try to be a good person. I stand up for myself when someone wrongs me because I am worth it. Every action I take towards those values helps reinforce that I AM a good person, and helps me like myself. I don't let myself dwell on my imperfections. I just look at those as challenges that I need to work on.