Lots of good advice above. I have danced since a small child, and thought I knew quite a lot of dances inside-out and upside-down, but it's a very different skill from just doing the dances to calling them. For one thing, your mind has to be at least one jump ahead to anticipate the next move or figure in the dance. So here's my 5.
1. Keep it simple, and, hopefully, unambiguous: it's amazing what strange interpretations some people will put on your seemingly unequivocal instructions! (Once I told dancers to "fall back into place" - one young man lay on his back on the floor!)
2. Don't assume anything: "set to your partner" means nothing to complete novices: demonstrate what you mean and/or use your demo couple (great advice!): actions speak louder than words.
3. Throw away the clock! In these days of digital timepieces fewer people seem to understand "clockwise" and "anticlockwise". Just hope they understand left and right.
4. Terminology again: Count in steps, not bars of music. One caller I know tells people to dance for 4 bars which is actually 8 steps. He also tells people to "polka round" in a waltz, and "waltz around" in a Gay Gordons.
5. Keep your cool and be pleasant even when they're screaming their heads off at each other while you're trying to teach a walk-through. It's their night out, not yours, so appeal politely for a little calm and hush.
And if I'm allowed a no 6: stop calling once it's obvious that they've got it: no need to kept blaring on to the end if the dance if they're doing it fine.