Is your fabric folded correctly? If your strips are buckled, try watching Leah Day's Youtube tute on folding fabric. She helped me get straight strips. The only thing that I do different, is after I get the first fold, I carefully press a crease and then fold it again, making sure that I have it hanging straight. If you watch her this will make sense. I also put little sandpaper dots on the back of my rulers to keep them from sliding. I don't know about the other product that quiltstringz suggested, but I'm sure it is close to the same thing.
I also have a Shape cut ruler and it does help a lot, especially if I am cutting up a bunch of strips.
I would also suggest practicing on a throw away fabric...cheap stuff that doesn't matter if you throw it away. Practice is the best thing.
Good luck. Gina