I am a real fan of clear plastic first aid tape. (I use NexCare brand)
When I used sandpaper dots they were always in the wrong place. With the tape you can see through it. It is flat so it doesn't raise the ruler up like the clear plastic gripper dots. It is also cheap and easy to use. <G>
Practice is the real key to getting comfortable with the cutter/mat/ruler combination. You might see if a more experienced quilter could watch as you cut to give suggestions.
Have fun, relax,
Pati, in Phx