First, Have iFile, Backgrounder (search it in Cydia) and, Cydia.
Next, open iFile and go to "Var/Cache/Apt/Archives".
Now, exit iFile and open Backgrounder. Select "Overides" and then "add"
Find Cydia, and add it. Now, click on Cydia (in Backgrounder) and then the black spinner.
Next, select even if unsupported.
Find the .deb file you want and install.
When it is finished, don't exit Cydia yet.
If on 4.0 and up, double click the home button and select iFile.
Otherwize, Cydia is running in the multitask on foreground. (Warning, device will go SLOW)
In iFile, look for the file. Move it!!!
There you go. Now you can install it without Internet!!!