This list is specifically for the PITS board; for more general advice about launching a weather balloon, see http://www.daveakerman.com/?p=1732.
To avoid runing out of space on the SD card during the flight, be sure to delete any photos stored during testing. To do this, type these commands into an ssh terminal window, or on the Pi keyboard:
sudo rm -rf images
Delete log files
The log files are unlikely to take up a lot of space, however it makes things easier afterwards if they only contain data from the actual flight. To delete them:
Backup The SD Card
Use an SD imaging program (same as you used to create the SD image in the first place) to copy the SD to an image file, and then to a spare SD card. Take the spare card to the launch site in case it is needed.