Best way is to do it online.
But as stated already you can NOT do it online if your passport is over 5 years old, so this isn't an option for over half of us.

I took a copy (which enabled me to check it when it got back).
I also rang up the DVLA beforehand to check that they had my motorcycle entitlement on their database. This means even if my licence was lost in the post that it would still be on their database (their telehphone systems is a nightmare though, keeps putting you through endless menus).

If your passport is over 5 years old you have no option but to send it off whether directly yourself or via the post office.