One thing for me is that I do NOT drop my feed dogs when fmq. Gasp!
I know. One of the classes I took when starting out the instructor was emphatic you cannot under any circumstances fmq unless you drop your feed dogs. Wrong! After forgetting one time to drop them, then experimenting, I found if I leave them up and lower the presser foot tension, it really helps my stitch length be more regular. I go much slower than most people do when fmq but, it's what works for me.