I;m with the group that says starch your fabric. I find that when I do this I get the best results. Good Luck. But if you can afford a larger mat by ALL means do it!!

Having a rotary cutter that easily cuts through 12 layers of fabric sounds like a dream. Where do you buy Martelli supplies?

My technique isn't the greatest, and perhaps one of the many reasons is that I would buy a roll of pre-cut fabric (which seems to come in 2 1/2" widths) and change the pattern to fit that size. I'm old, and life is just too short to fiddle with details that could be jiggled around.

Lots of good comments in this thread!

I have a 39" mat and fold in half and and in half again. I always got the dreaded "V" in my strips. I found out that if I iron the fold, the cut will be straight. I think this is a very good tip.

I just fold it in half like everyone else

I also use one or two hand clamps to help stabilize the ruler and hold it tight.

Marysewfun

When I have to fold, I use straight pins to hold the selvages together too. It gives a little more stability. Definitely invest in a larger mat and a 24 inch long ruler. I love the Olipfa 6" X 24" ruler to use as my cutting edge because it grips the edge of the cutting mat and gives stability as well.

I use Omnigrid rulers to actually measure though.

Rip it. Then iron the edges flat. The strips will be true.

I fold selvages together, then selvages to fold. I put the end with the selvages facing toward me -- the fold away. I was in a class and the teacher (who is nationally known) said this helps keep the wobbles away. The rest of the things I learned at the class were the all of the great techniques for straightening the fabric -- don't cut a yard of fabric without straightening.

That said -- if I need long strips parallel to the selvages (length wise grains), I always rip it. I don't rip wof (crossgrain) because some fabrics don't tear well crosswise.