I've been using Grangers wash and proofer 2in1 every time but it's an expensive way to do it.
Do you lot use fancy stuff all the time and if you don't what do you do?
Read the eVent website. IIRC it tells you to use normal detergent +/- some surface spray proofing thing afterwards.
I use nikwax techwash in a big bendy bucket and spray with TX direct afterwards.
Apparently soap flakes works well for breathable waterproofs too.
If the grangers stuff works I would perhaps keep on with it - as reports of people trashing their event stuff are rife.
I think the current thing for event is to wash with detergent to kill greases/sweat, then soap and then a DWR application.
(the grangers stuff has always worked well on my paclite stuff as well)
Nikwax Techwash.
After every 4 or 5 washes I use the Nikwax wash-in reproofer as well but it certainly doesn't need it after every single wash.