How to Download Deb Files From Cydia
First thing first, your ioS devices must already been jailbreak. You can use Sn0wbreeze or Redsn0w.
1: Use FTP to connect to your iPhone, or using other similar utility tool.
2: Head to this path: /private/var/cache/apt/archives
3: Now you can download any tweak or app from Cydia, however it's not recommended to reboot or restart springboard.
4: Reload this path /private/var/cache/apt/archives folder, then .deb file will appear and you can download it anytime you want.
From now and then, you have backup for all your Cydia tweaks and apps saved in your computer.