[Looka]

Yup Toothpaste... and do it a few times..

Not sure if it helps but on occasions i still get a tiny bit of fogging u/w and if that happens i usually let in a small amount of water into the mask and just roll it about inside to wet the fogged areas... then purge it. (Mind you I’m assuming its a single lens mask)

Not sure if that's standard but it has worked for me as i find even the slightest fogging irritating..