Difference between required and too much interest?

I always see people suggesting not to show interest, but I know this is false, as most of the girls I've slept with, I contacted months after I first met them. This is always showing at least some interest. So what's the difference between showing the required amount and too much interest, especially as far as messaging goes? I say, write enough to flirt and make plans.

Edit: also, what conversation starters work for you?