Free miles? Probably not what you are asking, but before I do just about anything, I check to see if I can get miles for it. I consider this "free" since I was going to do it anyway.

My best example was getting Lasik. I was going to do it. Searched and found that Delta gave 25k miles for a surgery (technically 5k for the free analysis and 20k for the surgery). I lucked out that it was through the same center that offered me 15% discount due to partnership with my eye insurance company.

Off the top of my head, things I would check for free miles before doing:

Opening a bank account
Changing cable/satellite provider
Opening an investment account
Getting a mortgage
Refinancing a mortgage
Getting any subscription service (Netflix, etc.)
Renting a Car
Buying a Car

Obviously bonus miles are the least of my worries in some of those items, but it never hurts to see if someone will give you miles to do what you were already going to do.

Just like when I got me peepers cut!