I have a small duffel with spare clothes I keep in my truck. I didn't go out of my way to buy cheap clothes. A couple pair of quality wool socks & liners is a good start, followed by a pair of jeans that are well worn in and comfortable. If you're in a humid climate, think about materials that wick. For shoes think about replacing your hiking boots while they still have lots of life and putting the comfortable pair that are just right into reserve status.

My philosophy on spares is wrapped around the idea that whatever makes you go to spares/back-ups may be serious and that is no time to be wearing cheap Wal-Mart specials that may not cut it in your hour of need. Don't set yourself up to fail. Don't forget a good belt and you may need a spare multi-tool also.