Here is a good article which describes how to pick a good password for your account at a web site.
Note that when picking the Memengo Wallet encryption key you need to follow these guidelines which are more strict. The reason for the difference is that the encrypted data is stored on your iPhone, not on some remote web site. Trying out all possible passwords on remote web site is naturally limited by the network speed, and may indeed only work at a rate of about 100 passwords per second, however someone who stole or found your lost iPhone may be able to try out the passwords at a much higher rate.
