1) Utter the words, "Honey, I think I'm good with our (1, 2, 3...) kids.
2) Upgrade to a nicer car or house that is just perfect for the size family you currently have. Just make sure when doing this that you are completely strapped in with your note and can't afford to upgrade again for another 5 years.
3) Schedule an appointment for a vasectomy. It's like those sperm KNOW this is their one last chance to make a baby. They will tear through any condoms or birth control you may be using before the big day.
4) Half-ass chart your cycle. Oh you've ovulated already this month? So you think you are safe? Well surprise! You overachieved and ovulated again!!!
5) Don't consider the fact that your husband may have super sperm and that those bad boys may live for 6+ days.
All of the above have worked wonderfully well for me. To significantly increase your chances of getting pregnant, try two or more of any of the above.