Yes, you definitely need to put something on the bottom of your ruler to keep from sliding. I used the little round sand paper looking circles I got from JoAnn's.
When you fold your fabric -- put selvadge edges together and fold, making sure fabric is straight (no wrinkles, etc.) Fold again toward the selvadge smoothing out all wrinkles. Now, you need to cut the fabric so that the edge is perfectly straight. After you do this, now begin to cut your strips. And guess what -- no "elbows" in the strips.
After cutting a few strips, recheck your straight edge and trim as necessary to keep it straight.
I saw this on Fons & Porter and it works for me!!