I have saved my used rotary cutter blades for many years in hope that someone would develop a way for me to sharpen them. I've tried a few but they were not very successful.
The most recent one I found is the TrueCut Linear Rotary Blade Sharpener by The Grace Company (www.graceframe.com).
It promises to 'Extend the life of your blade'. The sharpener comes with detailed instructions for sharpening. You don't have to hold it in place when sharpening so there is added safety.
I tested one of my saved blades and then cut a double layer, then 4 layers and then 6 layers. I had no trouble cutting 2 and 4 but cutting 6 was a bit more of a struggle. This tool will extend the life of my blades and I will use it next time I find myself having to press harder to cut. They also make a motorized sharpener so you can sharpen your blades with the push of a button!