I've posted a rather involved lineup change scenario with DP/FLEX; Steve M suggested printing out lineup cards to work through the options.
This has been posted before, but I just thought I'd mention it again. Cactus Umpires has a powerpoint presentation on lineup card management, including a discussion of DP/FLEX.
It looks like a well thought out system. I've not used this system myself, but I may give it a try this year.
Go to
Cactus Umpires and click on the link "FREE Download! Lineup Management System for ASA & Federation."
With my browser setup, I have to right-click and download the presentation, and run it locally on my PC. That's best anyway, 'cause you'll probably want to go through it more than once.