Jane Quilter

And cutting dies are repetitively accurate if you are cutting a lot of one shape. See https://www.accuquilt.com/

NJ Quilter

You've gotten excellent suggestions from everyone so far. I'll be so bold as to do a quick re-cap in no particular order.

-Put your index finger on the top edge of your cutter
-Use at least a small piece of painter's tape on whichever side of the line on your ruler you decide is where you want to cut. (I've advocated this for a long time and only recently started practicing what I preach - way too easy to have not done so sooner).
-Use your ruler to measure vs your mat. If you are having to make cuts larger than your ruler width, I'd do a slightly larger cut and trim up my unit/block as needed. You can butt 2 rulers together but I personally have not had much luck with this method.
-Keep one finger off the ruler onto the fabric to help hold the ruler steady. 'Walk' your hand up the ruler as you cut, keeping that pinkie finger on the fabric and the rest of the fingers on the ruler the whole time (sounds harder than it is).
-Use the same brand ruler throughout your project. If you need to re-purchase rulers along the way to accomplish this, it's well worth the expense - but use a coupon!
-Check your table height. There are lots of 'erognomic' sites out there to give you hints as to how to measure the best table height compared to your height. If you are stretching a lot to make cuts, likely the table is too high for your body height.
-I'd also recommend getting some of the clear rubber sticky dots to place on the 4 corners of all of your rulers. It helps grip the fabric and reduces chances of slippage.
-Slow down and take your time. Take a deep breath, relax!

QuiltingVagabond

A tip from Bonnie Hunter that helped me a lot is to place your ruler on the fabric so that the measuring line is on the fabric not next to it. Those couple extra threads in the cut piece can make a big difference in getting the correct unit size.

stilllearning

If possible, don't cut through too many layers. My cuts are more accurate when I don't cut through more than 4 layers, preferably only 2 layers. I'm waiting until I master the accuracy aspect before I worry about being more efficient or fast. If all the layers aren't folded perfectly, the folded areas will come out wonky. Measuring with the ruler instead of the mat seems to work better for me. Also, I've had better cutting results when I starch the fabric during ironing, especially since I pre-wash.

QuiltnLady1

When I was learning to use a rotary cutter and ruler, I watched someone on Simply quilts who used two rulers. One ruler measured the width that was needed and a second ruler was then butted up against it and the second ruler was the one you cut against. I use the ruler on the right to measure and the ruler on the left to cut -- I actually lay my forearm on the cutting ruler because I have hand and wrist problems and can't hold the ruler with my hand. It works well, but DH got me the Accuquilt Go Big for Christmas and it is a dream for the dies I have -- I just have to make sure the pieces are big enough to cover the blades (they don't need to be straight) and the pieces come out perfect every time. For the Go, I do still starch.

Tartan

​Use the lines on the ruler. Have the line on the fabric when cutting next to it. Use the same brand of ruler throughout your cutting.

awf60

Thanks everyone for the wonderful advice!

Battle Axe

To keep my ruler from slipping, I am applying some Elmer's cement glue to the back and letting that dry overnight. It's just a little tacky, just enough.

The Creative Grid rulers have etched spot on their backs and don't slip as much.

Don't cut with a person who is hard of hearing and wants you to look at them when you say something. I looked up from my cutting and sliced off the edge of my left index finger. What a mess!

The person was so rattled that she could not help me. We finally found a doctor and he said I had not cut enough off, not enough to sew back on. Took forever to heal but I did miss the nail.
Marcia

railroad

This has helped me, too.

feline fanatic

One thing I have not seen mentioned and a common problem is make sure your fabric is folded straight. You will know it is not when your cut strips come out with a dog leg (V notch) in them on the fold lines. Rarely is fabric cut straight off the bolt, heck rarely is it even loaded on the bolt straight from the factory. So when you line up your selvages you may have to offset the cut edge as much as in inch (sometimes even more!) to get the fabric straight.

Here is a good youtube by Leah Day
https://www.youtube.com/watch?v=mkHLWTQSy1M