Always ask someone out for a specific thing. Make it easy on yourself by its being something that you are doing anyway. "My friend just gave me two tickets to the Hoya's game Wednesday." "I'm going to a sneak preview of "Evita" on Friday and I've been invited with an escort." "A bunch of us are going to Happy Hour tonight." First of all, you won't be a nervous wreck asking somone out this way. After all you could be asking them to come along out of friendship. Second, they can say yes, or they can say, "Sorry, I already have plans for Wednesday" without it being a whole issue. Either way you've just gotten a whole lot of information without anyone putting themselves out on the line, which would have made everybody nervous later.
And never ask more than once. If the person says no, they said no. (Unless they made it clear that they genuinely had a previous commitment and would have been otherwise interested: "Wow, I love basketball and want to be first on your list the next time you get tickets.")

Personally, I'm very big on the Happy Hour group date. I think when you're first interested in someone, you should play it very low-pressure. It's more of a "I like you and want to hang out with you" thing. Get them out with your friends. Spend some casual time together. Then when it's clear you like each other, get them alone. But if you start with a date date there's so much pressure. It's a job interview. You have to decide at the end of the evening whether you like them, whether you want to go out with them again. I know of very few relationships that start from real Do-You-Want-To-Get-Dinner-With-Me-Saturday-Night dates. Much better to get to know each other on a couple of casual outings. You'll be friends first, which is always a good thing in a relationship. And if one or the other of you is not interested, then you find that out before it becomes an All or Nothing situation.