For straight lines you have to pivot on your elbow or pretty long lines use your whole arm.
If you pivot on your elbow you have a slight curve obviously but you have to correct that curve along the way you know. Watch scott robertson's dvds and you will see that you can pull of a perfect straight line while pivoting on you'r elbow. Its a matter of confidence and practice.
Just keep connecting 2 dots and put them further apart each time.