I was in the hot tub last night and this wonderfully fun girl named Melisa asked me just this question (i'm not sure why the guy she wants to ask out doesn't just grow a pair...of eyes to see what is right in front of him and ask her out himself!).
To me, there are two good ways to ask a guy out and two very bad ways.
Good way 1: find some obscure place that you KNOW this guy has never been to and say, "Hey, [insert name of victim], have you ever heard of [insert obscure location for eating/doing something fun (note: not squaw peak)]? [pause]...No? (sound surprised...practice in the mirror, even) We should totally go there sometime." Classic. Simple. If he doesn't ask you out after that...well, you gave it a good shot and move on. He either doesn't like you or is too dense.
Good way 2: "Hey [insert name of next victim], it is wednesday night! That means 2 for 1 pie night at Village Inn!! Come on! Pie's on me!" (i'm still waiting for the day when someone will say this to me...[*sigh])
Bay way 1: "Do you want to go out with me?" Ugh. Let's face it, a lot of guys like this just because they're wimps. Give hints, be nice, invite them over for dinner, do the first 2 ways...but seriously? It is fine to ask a guy out if he has already asked you out, but be patient. Play by the rules. Let him work for you. You are worth it.
Bad way 2: "I have this ticket to this thing and this guy just bailed. It is in like an hour (and i know that you OBVIOUSLY have nothing to do)...do you want to come? (oh yeah, and don't bring your self respect...you won't be needing it tonight)." BOOOOO! Ask a friend who is gay...not a guy you're interested in.
So yes, it is okay for a girl to ask a guy out, and we would LOVE it if girls would ask us out a little bit more...just not in such a scary way. Hunters fear being hunted. But in the end, if things don't work out, at least you won't be in the friend zone anymore. Phweewh!