What you're looking for is called Speed on Demand. SoD. It basically sets up your movement keys to activate Sprint (or SS or Fly) when you press it and deactivate it when you let go. It's not perfect, but very handy.
CityBinder Beta Version 0.3
CityBinder has built in SoD to help you easily set it up.
If that's too tricky (it was for me for a few months) then use this.
/bind g "powexectoggleoff sprint"
Replace G with your key of choice. Pressing G will ALWAYS turn off sprint.
I am tired of my low level toons running out of end because i forgot to turn off Sprint during a cave mish in the Hollows. Can a bind guru point me in the proper direction or tell me how to set up a sprint toggle on/off bind connected to the movement keys?