Women are like plants. You've got to nurture them to keep them healthy. If you don't they will more than likely gravitate to someone who does give them attention.
I'm not saying be just call to see how they are doing, call them to tell them what YOU are doing. They enjoy some type of engagement with you even if they aren't actively a part of it.
This is extremely important if you are seeing multiple women at once. You can't be with all of them all of the time so that is why its imperative that you reach out and touch them via phone or text periodically. Even though you aren't seeing them often, you have to make them feel as if you are to keep them interested.
In the long run it's easier to have them hang out with you on your schedule when they feel that you aren't just calling them because you don't have a date. This may be different for teenie boppers who don't have a social calendar, but for a woman that can easily find something else to do it makes sense not to show them that they are just a 'fill in.'