I keep a complete set of clothes for 1 adult in our family BOB in my Jeep. (Socks, unders, LS t-shirt, fleece pants, rain pants and wool jacket.) They're not all fancy and they'll be wrinkly but they are clean and dry.

DS has a complete set in his survival kit, which also lives in the Jeep. Having an adventurous 8 year old boy means that his spare clothes have gotten a work out on several occassions. What a relief it is to get arrive in the parking lot after a hike or something, him covered in mud, and be able to clean him up before he gets in!