I second what gothgirl said; but in case you were looking for more concrete advice:

1. Exchange compliments. Accept compliments graciously but modestly (e.g. "Thanks, that's kind of you").
2. Increase your sense of humour (watching comedy, reading jokes) until it's at population average levels, if it's not there.
3. Practise considering other people's point of view.
4. Do those three things consciously for long enough that you start doing them automatically.

At least, that's what worked for me when I was younger. Especially 1 actually, I think it helped with 3.