A couple things;
1) Make your intent to get her on a date CLEAR before grabbing a girls number. Something as simple as "Let's grab coffee soon" gets the job done because, at that point, she's making the decision to go on a date with you. If you grab a number and she has no idea what you're looking for then you're very likely going to meet a lot of resistance.
2) I prefer 2-3 replies between the two of you before asking her out for the first time. Girls, typically, meet a lot of people on any given day so assume she doesn't remember exactly who you are SO your first few messages should help her to remember you.
3) Keep it simple, as Tony said

Although, unless you're in a position where you basically only have one day and time available then I find it best to keep your text flexible. "I'm thinking we should grab that coffee this week. What's your schedule like?

" Going with something like "Let's grab coffee next Wednesday at 9:30" is very limiting and it feels like an unnecessary risk to take because if she's not free that day AND doesn't offer a different time slot then you're gonna fight an uphill battle.
-Richard