1) Open a command prompt with admin privileges (search for cmd and then press ctrl+shift+enter).

2) Enter the following command:
Code:
bcdedit /enum /v
3) Select (right click -> mark -> click and drag) and copy (right click on highlighted text) the identifier number (with brackets) for Windows Developer Preview.

4) Create a new boot option with the command:
Code:
Bcdedit /copy {IDnumber} /d “Windows 8 Safe Mode”
5) Start the configuration utility by typing "msconfig".

6) Go to the boot tab -> Select the new entry you created earlier.

7) Mark Safe boot and set it to minimal.

8) Click the checkbox besides "Make all boot settings permanent" and click OK (you can also change the time delay if you wish, default is 30 seconds).

9) Reboot to see the new entry.

Source: http://www.blogsdna.com/18209/how-to...er-preview.htm