Don't just offer a generic invitation: 'Want to go out with me sometime?' That makes it really hard for someone to say no. Which you might think is a good thing--but it's not. It sets you up for an evening with someone who doesn't want to be there, who's kicking themselves for not having figured out a way to say no nicely and who might have actually been psyched to date you if only you'd gotten to know them a little first without all the pressure. Thus DO NOT presume that the other person wants to go out with you. It might just be the other way around hence don't create an awkward situation for you and of course for the other person.

Always ask someone out for a specific thing. Make it easy on yourself also by making the rendezvous in such a way that you were going to do it anyways. Like inviting him/her to your house warming party or B'day party or any other party. Or else you can invite him/her to some get-together or picnic. First of all, you won't be a nervous wreck asking someone 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, without it being a whole issue. This way you will ensure that you are not embarrassed and also make you know the person a bit more.

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 watching Brad Pitt movie and want to be first on your list the next time you get tickets.')