[jamie_1930]

I would use this

if(joystick.joy1_y1 == 0 && motorspeed > 0)
{
motorspeed = motorspeed - (desired decrement)

}

it's simple and simplicity is the key. So what is basically happening in this code is that if you've let go of the left joystick and the motor hasn't stopped then it will decrease the speed then the rest of the code will execute then the process will repeat. The only tricky thing about this is the decrement will have to be small because the code will repeat almost immediately.