This method is pretty much standard for Samsung phones, but much credit to josby at XDA for verifying it works with the Sidekick
Read this. It very much applies here. If you're good to go after that:
This can screw things up badly. Once you decide to try anything that T-Mobile hasn't approved, whatever happens is your responsibility. Not Android Central's. Not mine. Especially not mine. This is in a big red box, so don't even try to say you didn't see it.
Install the Samsung USB drivers and Android SDK. If you can't get those up and running, you should probably quit now before you break anything.
Download both attachments, and unzip them. Place the files "rageagainstthecage-arm5.bin", "su", and "Superuser.apk" in your Android SDK/platform-tools folder. Credit for these files goes to their original authors. Source code for RATC is included in the zip file.
open a command window
cd to your SDK/tools folder
reboot your sidekick
plug it into a full speed, primary, USB 2.0 port on your computer.
in the command window, enter the following, one line at a time: