Been "a while" since I've done this on the bicycle but a backpack is out, get a good seatpack as znomit suggests and take a tip from the LD/IBA motorcycle community. When we have a fixed route ( which encompasses a good bit of North America ) UPS fresh off the bike clothing ahead to your reserved motel/checkpoint, discard and proceed. Why burden yourself? You must have whiteys, pants, sox & tees that have seen better days but are presentable enough but disposable. While everyone else is scrambling to wash/dry a whole load you can be relaxing with a nice beverage after washing only 1 of 2 jerseys/shorts in the sink to dry overnight. But pack your flip-flops, rain gear etc.

Back in earlier ages this sort of cycling was served by traditional makers like Carradice, they knew/know what a proper gent's needs were/are:

http://www.carradice.co.uk/index.php...&product_id=41

Complete maintenance & reasonably fresh tires are assumed, pack for 1,000mi. of flat repair (2X planned) and take a full sized frame pump. Cell phones need chargers, check the group & take 1 per device type. Less is more.

-Bandera