Pica Pica, like the weather, is unpredictable. It depends on the temperature of the water. Just like cats and dogs, the jelly fish seem to 'come into heat.' Some years it is as early as April and others as late as June.
As the ocean temperature heats up those little Thimble Jelly's start laying their eggs.
So, it is really hard to out figure it. By the same token, you can dive at one dive site and not have any and a little ways away have a lot. These larvae drift with the tides.
Everything that has been mentioned works - sometimes. Think of it like Poison Oak or Poison Ivy - if you go into the woods it will probably be there! JUST BEWARE!
Grease up -
Wash off -
Wash your swim suits - latex skins - wet suits well with soapy water and hang in the sun for a short period of time.
Here is an example of where it is better to get pissed on than pissed off. (sorry - couldn't help myself)
RELAX - DON'T WORRY! Either you get it or you don't