One of those times when Jane Austen demonstrates the opposite of advice

[The surrogate father] wished [the wannabe lover] to be a model of constancy; and fancied the best means of effecting it would be by not trying him too long.

- Mansfield Park, Chapter 35

Yup. When you're concerned about the callow youth's tendency to be fickle in love, the best way to avoid a problem is to not give him time to change his mind.

Leila picked up the book around the same time I did, so check in at Bookshelves of Doom for her updates.