How do I get my Sony Ericsson K810i to go on Standby?

[platyguin3000]

I recently bought a second hand k810i, and am really pleased with it, except for 1 thing. Apparently if you hold the "back" button down, it will go into standby, no matter how long i hold down the buttons for the phone remains turned on, I have to wait for it to automatically go to sleep. How can i fix this?