here's an example:
on the onClipEvent (load) thing i create 3 variables: xM (stands for x-movement and yM for y-movement, speed is the movespeed)
xM = 0;
yM = 0;
speed = 5;
and then on the EnterFrame thing:
xM = Math.sin(_rotation*(Math.PI/180))*speed;
yM = Math.cos(_rotation*(Math.PI/180))*speed*-1;
_x += xM;
_y += yM;
this way, it moves in the direction it faces with the speed you defined..
i hope this helped more than Nano256's comment.. ;)