One trick that worked for me was:

over inflating my tyres,
riding too fast on a wet path,
riding too fast over the wet, rubberised cover on one corner of the bike path,
going down,
bike behind me panics and goes down,
the three riders behind him all crash in a big mess of arms, legs and spokes
it took them so long to get up, there was no more drafting

BTW no major injuries, just lost bark