When a specific dance is announced:
If you know a dance well, dance on the perimeter of the dance floor so others can follow you.
If you have just recently learned the dance, go the the centre of the floor before the song begins, so that you can receive some assistance from the folks around you.
Especially watch the outside corners if corner leaders are in place.
If you need a couple of minutes to recall the dance, watch from the outside and join the outside lines when you are ready.
When there is no announced dance:
Make a collective decision with your fellow line dancers about the dance appropriate for the music. Then take your places and join the music together as you can.
When “partner dancers” are on the floor:
Generally, at social dances, the line dancers take the middle of the dance floor to allow partner dancers to dance around the perimeter — unless the floor has a designated line dance area.
If you MUST leave the floor before a dance ends:
Exit via the shortest path, and be VERY cautious of your dancing friends.