I've never been asked out by someone who already knew me a little - oh dear, does that say something bad about me?! 
It would depend how well they knew me - what "a little" means.
From anyone, a "would you like to go out for a drink with me on Saturday?" would be just fine.
If they knew me well enough to know my interests a bit, is probably be happy if the suggestion of a meet up was tailored.
"I was wondering if you'd like to play squash with me on Saturday, and get a drink together after?" would work well. But if I fancied them, just the drink would do.
What WOULDN'T work is any old crap about being physically attractive added in. Ha! Just realised this is relevant to your other thread
But honestly - my experience is OD more than people I already know, but I am very
about physical comments ("you have beautiful eyes"
) because it's not central to who I am.
General rule: keep in low key, elaborate, simple - no pressure. Not in public. It's OK I think to ask by text or email too, if you want to avoid the pressure of in person.