It's generally recommended by most security experts - and in turn by companies who provide webmail services - that you should change the password of an email account on a regular basis: once a month, or once a week for those with a little paranoia.
When choosing the syntax for a password, it's important you do not do the following,
It's recommended, when picking a password, to pick a jumbled up sequence of letters and numbers; the letters being a mix of lower and upper case characters. The longer the password the better; most webmail services require a password which is 6-8 characters in length, although it is probably wise to select one which is longer. An example of a password could be something such as: aBB71kPo57nb9.
Basically, a long password, which has no discernible meaning is much harder to crack than one which is short and does; especially if it has a personal meaning to the user. Of course, short passwords which do have a meaning, are much easier to remember. There are password storage programs, such as Roboform, which store and input passwords into login forms - ideal for difficult to remember passwords.