Sidenote: This isn't something I made up, this is what a lot of people involved in the world of dating use to sort out the people who really want to see you and spend time with you, from the people who are going to waste your time, and money.
What is the 'Takeaway'?
In sales, if a customer is reluctant to buy a product, the best thing you can do is say:
"The price is set to increase next week after our sales, if you change your mind, I'll keep this discount voucher for you".
You offer something (in your case a date), they decline (flakey/rejection/blow off), you withdraw the offer (the 'Takeaway'). Simple.
Now the best way to do this in the context of dating, is something along the lines of:
"No problem :) We'll just do it some other time"
This will reveal the interest she/he has in you and how much she/he values you and your time, but only if they retract what they initially replied to you.
If they say anything like "OK we'll do it some other time", they're not interested.
Use the 'Takeaway' if someone replies things "We can meet up but call me on Monday to set it" or "I don't know my schedule for next week"
It can't hurt to try it out?