For Factory Reset
If you are looking to start over again and wipe all data and apps then you need to do factory reset. For that follow these steps
Menu button> Settings > SD Card and Phone Storage> Factory data reset> Reset Phone.
For Hard reboot
Take the battery out and put it back in about 10 secs. In this way you will not loose your data or any installed apps.
How To Reset G1 – Wipe Corrupted T-Mobile HTC G1 Phone – Easily Restore To Factory
How to Hard Reset the Tmobile Wing
factory reset T-mobile G1 phone
Power off the G1
Hold Home Key + End key for 20 seconds or until you see a “triangle with an exclamation point and a picture of the G1″
Open the QWERTY keyboard and hit Alt+W
Restored!
TO HARD RESET THE DASH 3G:
Remove the battery from the device to power the Dash 3G off.
Reinsert the battery, DO NOT power back on.
Press and hold the Volume Up button.
While continuing to hold Volume Up, press and hold the (red) Power/End button.
A multi-color screen (boot loader) will appear, continue to hold both keys.
The reset screen will appear (a grey screen with blue text).
Press Send to reset, then Send again when prompted to reboot the phone.
The device will reset and boot to the home screen.
