Combo Box Question (How to cycle through choices?)

[nakedbamboo]

I have a combo box where the user chooses the state. I would like to have it act like the combo box everyone is familiar with where if you hit the same letter, it cycles through the choices in that letter. For example, if I hit "O" twice I want it to cycle to Oklahoma instead of typing "OO" in the box, or if I hit it one more time, I want it to cycle to Oregon. How would I go about doing this in my combo box? Thanks.