When to boost a post?

I seem to remember reading last year an article about when to pay to boost a post's reach further out and gain new likes/followers.

It mentioned about first looking at seeing what natural reactions/reach your posts get, from the fans on your page and then choose the best of these. It makes sense really – the more popular a post is with your current fans, the more likely it is that that post will do the best when you pay to 'boost' it to more people.

Has anybody had any luck paying for a…

When to boost a post?