Use your left softkey or the * key depending on what program you are in. For example, when replying to a text message and you are not in t9 mode, hold down the asterick key until the menu pops up and then select t9. Usually it will then stay on by default.

There is also another setting in the registry and a couple of places where you can set the t9 to default, but this is usually the easiest way, since you have to switch out of t9 alot to put in websites or password, etc.