What about something like this? http://aztecmcu.com/catalog/i105.html
I use mine so much I have dedicated an older Duemilanove to it.
Putting an ISP connector onto your board is another way to go, but you still need a programmer. I would imagine you also need to allow for ISP in your circuit design. You could easily have things attached to the SPI pins that would prevent you from using them for ISP.