I'm about to get my P9 off of lay-a-way and I need a good way to carry my backup mag.
I thought about just putting it in my pocket but I would rather use a "proper" mag retention technique.
What advice does everyone have?
Thank you
Does no one carry spare magazines?

When I carry my P9, I carry two spare magazines in a DeSantis OWB magazine carrier.
http://www.jgsales.com/product_info.php/p/sig-p6-225-leather-single-magazine-pouch/products_id/2576
I bought 5 of these pouches from Centerfire Systems when they were $1 each. Unfortunately they are out of them now.
The 5 I received are like new, They fit my P9 mags perfectly.
Try searching for Sig P6 Mag pouch. You may be able to find them cheaper elsewhere.