Hey BL - just stick it in the sink (if you've taken it off) with some warm water & a little bit of washing up liquid. Use a non abrasive cloth to gently wash it.
If you haven't taken the visor off your lid, then using a wet wipe is a pretty good alternative .
Soak the visor in luke warm soapy water, if you go at it you will scratch it to pieces with bits of crusted on flies and road grime. Gently remove the bits onced soaked, with tissue paper soaked in the soapy water, just go really carefully or it will kill your visor straight away. Gentle rubbing on your helmet will keep it looking good
As a quick fix, I fold a couple of sheets of kitchen paper and pour on hot water, as hot as you can get it, then lay it on the visor, leave for a minute or so, then gently wipe it off. It should have time to disolve any 'nasties'
You dont need to take the visor off unless you have been licking the inside (which if you're speshul, like BJ you will need to do)
I usually just rest a few sopping wet kitchen towels on mine for about five mins and those pesky dried on flies wipe straight off, then dry off with one of those nice fluffy anti static micro fibre thingys