It very much depends on what you're wanting to do. If you're wanting a completely new wardrobe, then get rid of all but some basic t-shirts, tanks, underwear, and a couple old pairs of jeans for random things (never know when you could need 'em).
If you're wanting to just minimize your wardrobe, get rid of anything that you don't see yourself wearing within the next week/month. If you won't wear it within that time... I highly doubt you'll wear it later. Exceptions would be ultra-nice dresses and/or business suits. Those could come in handy at any time. Just keep them nicely stored away.
With the keep/toss/charity rule... what do you mean? With charity, my rule is this: don't give anything away to it that you wouldn't wear yourself (I'm not talking about style, but whether or not it's worn and/or trashy). Like... think about if you were in their shoes. Would you want someone's second-hand clothes that look like they've been dragged behind a truck and been run over a zillion times? So imagine you're in their shoes, and give them only what you yourself would want to have. Toss anything that you don't want, and that you don't think anyone else would want. You COULD however, re-use the cloth... if it's not bad quality.
For the most part... use discretion. And the week/month rule that I like to use.