Light your rack. If you load things on your rack, then a seatpost-mounted tail light isn't going to suffice. The Rack Blinky 5 comes with mounts for your rack and your seat post, so you can choose what's best for the situation.
Wait! There's more: It can also clip to your belt or your bag.
Busch & Müller Fork Crown Mount for IXON Speed and other Bike Headlights
Planet Bike Blaze 2 Watt LED Bike Headlight
Planet Bike Rack Bracket