First, you'll need to add the person to your workspace. Do this by going to your My Contacts section and entering his/her email address. Then add the person to a project from the Overview section of a project (simply click People tab on the top right of a project to add them). When you add them to a project, they will get a notice to register. They need to register first before taking over the account.
Then go to Account - Admins and add them as a System Admin. Simply type their name / email address to add them. Be sure to add them as a system admin, not a project admin. That gives them full system rights.
Once they are an admin, they can now control the account. They can remove you as an admin, or you can cancel your personal account. Do this by going to Account - Summary after you add the new admin. Click the cancel link at the bottom and follow the instructions to cancel your personal account.