I agree with most of what's been written...I also was more insecure in the past, with a poor self-image.
It's quite hard to change a deeply-held self image, and my subconscious resists that change a lot. For me, that's where "backsliding" comes from: my subconscious whispers, "...but remember, really you are not a lovable, sexually desirable man...really you're not...you're just fooling yourself when you believe she likes and wants you..."
But yeah, over time I have mostly defeated that nasty little goblin. (I wish I knew how he got started, but that's a job for psychotherapy.) There were a few things I had to tell myself to do:
1. Interact with other people in ways that affirm your self-worth. Don't be competitive and try to one-up people to bolster your ego; instead, ask them about themselves, draw them out, be a good friend and a considerate listener. When I do that I get welcoming smiles, warmth, and body-language that tells my emotional self that I'm a good guy despite whatever the goblin whispers.
2. Take care of yourself. You're worth enough to get some exercise and relaxation, to dress in clothes that make you feel good, to walk with your head up and back straight, to say what you want without apologizing. Like Rick writes, there's something of the "act like you're secure and after awhile you will BE secure" about this.
3. Talk to people about it. Almost every time I say "You know, I used to be terribly insecure and I still feel that way sometimes" someone else says, "Oh, God, me too!" That really helps -- it's not just a dark little personal secret, almost everyone has some of it.
I'm not convinced that cocky, take-charge assertiveness is what women (or other people) want as a dominant trait. Women have told me that a little shyness is charming, that honesty and transparency are appealing, and that "you can't go wrong by being gentle". But maybe that's just reflective of the kind of women I like, I dunno.
How about it, women? What percent mix of assertive, confident, shy, and gentle do you like in a guy?