Be direct, do it in person, don't specify a date/time until you've gotten a yes/no response.
NOTE: These suggestions are best for an aquaintance or a stranger - they're focused on getting a "yes" to the date offer so that you have at least an hour with the person to get to know them and show them that you're worth going on a second date with.
1. Be direct - if you ask me "Would you like to get coffee with me sometime?", it's a question that says "Would you like to spend time with me?". It's direct, and I have to answer. Don't give an her an easy out (e.g. "Are you free for coffee this weekend?" can be answered with "No"). If she's not sure how she feels about you and she has an easy out, she'll probably take it. If there's no easy out, she may say yes just to avoid being rude and think to herself "well, who knows, could be fun". I've gone on more than one date because the guy was direct and I couldn't easily wiggle out of the offer. I ended up dating one of them for quite awhile :)
2. Do it in person - you cannot see a person's face over IM or email, or the phone. IM and email are too easy to ignore, and honestly, seem a little cowardly. Phoning is a decent backup, but there's no replacement for the brave, in-person, can-see-your-face ask out. I bet it at least doubles your odds of a "yes".
3. Don't specify a date/time - if your ask out line includes a date/time (e.g. "Want to go to the art museum this weekend?"), you've just provided the easiest wiggle out ever. She says "Oh, I'm busy this weekend", and then you're left wondering whether you should ask "Well, do you want to go another time then?". It's a little awkward, and it leaves a question up in the air of whether she's actually busy this weekend, or whether she wiggled out of your offer.
The only reason I'm not accepting this one _yet_ is to see what else is contributed. This is the one to beat!