Both Odom's books have video on subnetting.
other videos are CBT Nuggets and Train Signal for CCNA
cisco game arcade has subnetting games
To add to Martin's comment above, you can check out the Games Arcade page here in the Cisco Learning Network where you can try subnet games (and more) that can be helpful in your learning experience.
Good luck in your studies!
Cisco Learning Network Moderator
This is a really nice guide:
Also you can use subnet calculator for quick checks: http://www.subnet-calculator.org/