Try not to take things so seriously. Not yourself, the other person, or the invitation. if you build it up too much in your mind, things don't go as well. "Hi" can be the toughest thing in the world to say, followed closely by whatever comes next, if you invest too much thought into it.
Next time you see an interesting person, too,take a breath and take in your surroundings, walk up to that person, say hi, and talk about what's around you (the band, the monster truck that just exploded, the lighting in the periodicals section of the library, the exhibit presentation, whatever). Say something positive, or ask him or her for an opinion on what's around you. Don't worry about a date - find out if you WANT to go on a date with that person. If something seems to click, ask if he/she would like to go out some time, to do something you now know you'd both probably enjoy. Otherwise, it's just a conversation, right?