You could say to those regulars that request it to invite you along to their next appointment with a dominatrix, so you can observe and learn the ropes a little bit - as a pro domme and apprentice domme session, so to speak. Chances are, they don't want anything particularly heavy, or they wouldn't be booking a vanilla escort in the first place. It would also put the ball back in their court, instead of putting pressure on you to provide services you're not familiar or not comfortable with.

Also, there are plenty of free clips online that'll allow you to develop the vocabulary and role play a bit.

A lot of the time clients who request services you don't advertise or provide just want to make you feel a bit inadequate or lacking in some way. It's odd how often the supposedly submissive ones are really more passive-agressive. A serious sub will look for a serious domme.

You could start weeding out the boys from the men by opening the door wearing a strap-on and a serious expression.   

Other times the outfit will do the trick - long black gloves and red lipstick and black lingerie are often enough to create the illusion. You can play with it and have fun.

Maybe consider joining FetLife and see if there are any workshops / classes / instructors in your area, or look up local dungeons and see if any of them offer classes.