If you don't ask, they can't say yes. This is the salesman's creed. Getting a date is all about sales, except that you are selling yourself. And be polite. If you are going to be real direct, use euphemisms for sex, don't just ask if they want to ****.
ed's advice is all good, I'm going to expand on this one (and I bet Thor can do even more, he's the king of salesgeeks on TB).
Don't just ASK for the sale, "Do you want to go out sometime/home with me tonight/to the parking lot?". Assume the sale, "So, when I finish packing up, are we going to your place or mine?". Be positive, not arrogant, is a key issue with this, nothing will run a girl off faster than being arrogant, but in the right situation, being confident/positive will be the best asset you have.
If you get a negative vibe, go slower and work up to the "sale", but don't ask until you can ASSUME the sale. If you don't get to that point, don't try to close it halfway ("Do you want to go out sometime...?"), better to leave it open ended if you don't think you can close it outright that night. Get the digits EVERY TIME, even if you don't get the impression you are going to close the deal, because in sales "no" just means they haven't been convinced to say "yes" yet.

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