I second most of the above. Lucky 7 is one I avoid. People
a)start counting at their partner (right) / or at the first persone they meet (wrong)
b)stop with no. 7 (right) / pass no 7 (wrong) (it's actually multiples of 6 that don't work)
c)listen to other people counting aloud instead counting people.

Keep it simple (and the simplify that for the first few dances). Give them a 'cheats option' e.g."the 1st couple need to get to the bottom and a strip the willow is fun - but get there anyway".
Have some flexible dances up your sleeve. If you ask for square sets and 5 or 6 couples stand up and it's the end of a long evening (after a long day for them) you may have no success getting more, so adapt the dance or change to another. Many longways dances can be done with varied numbers (I often stand them up in 1 or 2 long lines, then divide them).
Particularly at weddings you may find people wandering onto the dance floor after the walkthrough, OK for Blaydon races, maybe not for some dances. Be prepared for people needing partners found for them. And be flexible about who dances with who.
Remember you are THE ENTERTAINMENT. So if they want more music than dancing, give them that with good grace. If they overfill the dance floor, keep the momentum going so that they need a rest and make room for others.