[simme_]

    Just make a monthly recurring entry in your calendar that says "Check SSL certificates". 

If you rely on your calendar, it's simpler to create an entry in your calender for changing the certificate a few days prior to its expiration date. Monthly reminders will be ignored too easily.

[dice]

Better yet, add an SSL check to your monitoring system: open a ticket when it's going to expire in 30 days.

[zrail]

FYI I updated the post to mention monitoring systems.

[vacri]

A few days is probably cutting it a bit fine - what if you turn out to have an account problem with your cert provider?

[coherentpony]

s/days/weeks/g