Olga & Bob have given the essentials.
I can only add

1) Since people are there to have FUN I give them a motto "When in Doubt - CHEAT"
And then I give them a bit of advice on successful cheating.
e.g.
'if you think you are going the wrong way it's quicker to keep going you'll meet your partner and get home anyway' (grand chain)

'finish opposite your partner'

'turn random people till you get to the other end' (strip the willow)

This means that after a few easy dance I can do something more interesting. E.g. a strip the willow dance at a wedding (well, most weddings - you have to assess the crowd), or a complicated dance at club where most will manage but a few need more help

2.) If you've down something hard give them an easy or familiar one to relax and enjoy.

3.) Variety is good. tune types, energy level difficulty, dance shape.
e.g. if most of the wedding guests are young and energetic but there are some older I might call St. Bernard Waltz or another couple dance where they can come onto the floor and leave when it suits them.