I have found that a Master Reset does help but there will still remain files from programs that were deleted (just like most any Microsoft product). You can back up your contacts, text messages, photos, music and more using Microsoft MyPhone. You will need a Windows Live login which most people aready have.
Go to http://myphone.microsoft.com for more information. You will be directed to use your phone browser to go to http://myphone.microsoft.com/install and it will then download to your phone (if you have space) and walk you through saving your information. You can set the program to sync as often as you like. Your information will always be available on the http://myphone.microsoft.com site even if you delete it from your phone. You can remove it from the site at any time, of course.
An extra bonus, if you change or lose phones you can download the program (on a Windows Mobile phone) and sync your information to your new phone.
