I read a tip on here to put fabric tape in strips on the back. (The kind you use for bandages) works for me.
I use masking tape as I go, or if you wanted to leave it on for a few days, try using painters tape.
If you are using a 6" wide ruler, or one wide enough to do this... I've learned that not using finger tips but putting the full flat of your palm and fingers on the ruler to hold it works really well. It also allows you to cut for longer periods of time without stressing your hand or making you tired.
If you are having problem with you ruler shifting. That use to happen to me I bough a ruler from Martelli.com they are expensive but well worth every penny. It's made out of heavy rubber and it will not move have the different slits for different size strips and etc.
I feel your pain as I used to have the same problem. I purchased an Omni Edge ruler (using my Joann 60 per cent off coupon). But the ruler is much too long for me at 36". It has a gizmo (don't know what else to call it) on one end that hooks over the end of your cutting mat so your ruler can't move. However, I found the ruler much too long for me so I took the gizmo off (it just slips off) and put it on my 6 X 24 ruler that I use all the time. What a difference. The ruler can't move at all when cutting. I also have the dots, they worked OK. And I also have the plastic film and it worked OK too, but nothing works as well as the gizmo. Just do a google search for the OMNI EDGE ruler and you'll see what I mean. Too bad you can't buy just the gizmo. Someone would make a fortune.
In addition to the comment from copequilts, you need to gently walk your hand up the ruler. Place your hand at the bottom, cut about 8 inches, walk your hand up, cut another 8 inches, walk your hand up and cut to the end. Hopefully all this helps you.
Was fortunate to take a class with Nancy Johnson Srebro on rotary cutting. She started with the ergonomics, important to her since she had carpal tunnel surgery. She plants her hand, with her pinkie off the ruler and on the fabric, to act as an anchor. It really helped. I also learned that I was holding the cutter incorrectly, and straining my elbow. She demonstrated how your index finger should lie on the ridged area on top of the cutter, with palm facing down. I can cut for hours, and never end up in pain! She's a great instructor.
Thank you ladies I will try both suggestions
I find that leaning forward so that my weight is over the cutting mat and my arm points straight down rather than at a slant helps me a lot.
Bet you have small hands. I had the same problem and only practice helped. I also only cut 1 layer of fabric at a time till I had a better handle on things. I find if I spread my fingers across the ruler and press it helps a lot.The suction handle did not work for me.