My ability to convince my wife that my lens purchases were vital took a huge positive step when my son and daughter-in-law got married (I was #2 photographer for the event) and then ESPECIALLY when they announced a year later that they were expecting grandkid #1. Grandchildren are perhaps the greatest excuse to invest in good lens or upgrade what you've got. I highly recommend them.
I second this. And even though i'm too young to even have children yet, I tell my wife that I need to buy new lens well in advance of any children to practice and get my skill level up so that i can take those shots when she's pregnant. Also taking lots of photos of the new nephew in law and any family event helps a bunch too.
On the money side, I put away $100 bucks for "big ticket items" every month and that plus any extra money from my budget i use to buy a lens. Though after buying 4 new lens (1 being an L) in the last 3 months , going forward I'm going to try to limit myself to only 1 a year.
Just make sure you take lots of pictures of the significant other and make sure they are good helps a long way.