If you think "Master" is a bit too much at this stage, I'm presuming alternatives like "My Lord" or "Your Lordship" would be equally over-stretching it (or worse). On the other end of the scale, other "normal" forms of address to a superior such as "Boss" or "Chief" may sound tacky in a bdsm environment. Even though you feel a silly calling people Sir, a compromise may lie in qualifying it: "Kind Sir" or even "Dear Sir" - assuming your Dom doesn't object to being called kind! Last suggestion along those lines is possibly try "My Sir" - which to me conveys the feeling that He is special to you. As a random extra thought if either of you speak a foreign language the equivalent term in that language may feel good.

VV