These steps were initially published on AndroidAuthority here

We have done nothing more than used the instructions to verify they are OK and repeated them.  We have done this because these instructions can be hard to find, but we repeat, none of this is our own work and all props go to Chit Augustin at Android Authority.

Download the following files and save them to your computer:

• Revolutionary: S-OFF & Recovery Tool (Download from its website)
• HTC fastboot drivers.  Download the drivers and install them on your computer.
• ClockworkMod Recovery 4
• su-2.3.6.3-ef-signed.zip

Gaining S-OFF Using Revolutionary: S-OFF & Recovery Tool

Revolutionary: S-OFF & Recovery Tool is your best friend when it comes to unlocking the bootloader (S-OFF) and recovery of your device. Once installed on your phone, it will allow you to root it easily, explore various ROMs, and freely install custom kernels.  Follow the steps below to enable S-OFF on your HTC Sensation.

You may need to temporarily deactivate your antivirus and/or firewall software, as some security programs (e.g., Norton Antivirus) flag the Revolutionary: S-OFF & Recovery Tool as malware, even if it actually isn’t.

1. If you have HTC Sync installed, uninstall it, but leave the drivers.
2. Take note of your device’s serial number.  Pull out your phone’s battery and take a look at the label inside the battery compartment.  Your handset’s serial number should be printed there, usually beside the label “S/N”.  You will also find it on the box your handset came in.
3. Find out the HBOOT version on your handset.  With your phone turned off, press the Volume Down and Power buttons simultaneously for a few seconds.  The device will boot to a white screen showing device information, including the HBOOT version.  Take note of the HBOOT version number and reboot your phone after doing so.
4. Go to the Revolutionary: S-OFF & Recovery Tool website and download the appropriate package for your platform (i.e., Linux or Windows).  As soon as the package starts downloading, you will be redirected to a “Beta Key” form.  Fill in the form with your device’s serial number and HBOOT version number.  Press the “Generate Key” button to get your beta key.  Take note of this beta key.  You will be needing it when you run the Revolutionary tool.
   

• By this time, the package file for the Revolutionary tool should have finished downloading.  Extract the files into one folder on your computer’s hard drive.
• Enable “USB Debugging” from the Settings » Applications » Development screen on your handset.
• Connect the HTC Sensation to your computer via USB cable.  When asked for a connection type, choose “HTC Sync” and just ignore warning messages saying your phone could not find HTC Sync.
• Launch the Revolutionary tool (revolutionary.exe).  A command prompt window will open and will ask you for the beta key.
• Type (or better, copy-and-paste) the beta key for Revolutionary into the command prompt window.  The beta key is case-sensitive.  Press Enter to allow the tool to proceed.
• Wait a little bit for Revolutionary to enable S-OFF and unlock the rewritable NAND on your handset.

After the process is complete, you will also have ClockworkMod Recovery installed on your phone. However, in some instances, the recovery image does not get installed. If this is the case, manually install ClockworkMod Recovery as described in the next section.  Otherwise, skip to the next section on rooting your HTC Sensation permanently.

Installing ClockworkMod Recovery 4 on Your HTC Sensation

(only required if it was not done automatically in the last step)

Before you can permanently root your HTC Sensation, you need first to install ClockworkMod Recovery 4 on it. Usually, ClockworkMod Recovery also gets installed after you have successfully activated S-OFF on your device. But in some cases, it doesn’t.

ClockworkMod Recovery is a custom recovery for Android devices that enables you to execute several advanced recovery, restoration, installation, and maintenance operations that you can’t do using the stock recovery. It also allows you to flash unsigned files based on .zip packages and flash custom ROMs very easily.

To install ClockworkMod Recovery 4 on your HTC Sensation, follow these steps:

1. Download ClockworkMod Recovery from the link provided above.
2. Copy the PG58IMG file to the root of your SD card (i.e., the main folder of your SD card).
3. Turn off the phone and reboot into the bootloader/HBOOT screen.  To do that, disable the fast boot option first in Settings > Applications > Fast boot, then press and hold the Volume Down and Power buttons at the same time.
4. Once in bootloader mode, the phone will automatically detect the new file and will ask you if you wish to start the update. Press the Volume Up button to accept.
5. Once the file is flashed, simply reboot your phone.
6. As soon as the device has rebooted, delete the file from the SD card or rename it to something else, otherwise each time you attempt to go into bootloader, the phone will prompt you to install that file.

You should now have ClockworkMod Recovery 4 on your HTC Sensation.  Now, you’re ready to permanently root your HTC Sensation.

Rooting HTC Sensation Permanently

1. Turn off your device and reboot into ClockworkMod Recovery. (Disable the fast boot option first in settings then press and hold Volume down button while pressing Power at the same time. You will be in the bootloader mode where you can select Recovery to enter ClockworkMod Recovery.)
   
2. While in Recovery, create a Nandroid backup by navigating to Backup & Restore > backup.
3. If you have created a backup already, select “Install zip from sdcard” option, then select “Choose zip from sdcard”.
4. Select su-2.3.6.3-ef-signed.zip from the list. Then choose “Yes – install su-2.3.6.3-efgh-signed.zip” to install it.
5. Once installation is complete, reboot your handset.

You have now fully rooted your HTC Sensation. Your next possible step is to install a custom ROM and get rid of the stock ROM.