A good MASH username (or ID as Yahoo! refer to it) will help you when it comes to giving out your MASH URL to a friend and a well thought out password will keep you protected from people trying to hack into your account by guessing your password.
Tip 1: For usernames, try claim one that will be easily remembered by both you and friends, because the more someone can remember your ID, the less trouble they'll face while trying to get to your page!
Tip 2: For passwords, use as many types of characters as Yahoo! will allow you to. Usually a few letters and a number or two will keep you secure enough, but if they allow it, try to use non-alphanumerical characters such as an underscore _ to give you some extra security.