I'd like to add:
1. Make sure you are cutting from a height that works for you. Sometimes people think they can accurately cut sitting down. I can't.
2. Use your ruler to measure and not your mat. Mats are rarely accurate.
3. Always use the same ruler (or at least brands) for your cutting. There are differences.
4. Don't use those clear silicon dot things to stick on the bottom of the ruler. You just end up with wonky cuts from where the fabric stretches around those (ask me how I know). Sandpaper is a better option.
I'm sure other people have some more advice as well.