Today is again a day of sharing brilliant information found on Adobe Forums. Of-course, we shall not be copying the details of the formula shared. This is one of the technique which has been noticed at various places in Adobe Photoshop Elements Forums and proved very useful. I have tried it yet but feedback is quite good.
So if you are also looking for a solution to trim down your canvas to make your creation look good, check out following link - http://forums.adobe.com/message/4361259#4361259
Don't forget to say Thanks for Author(photodrawken) of the formula on same link !!