Nip to your local m/c accessories shop, or go online to something like M&P and get a little spray bottle of Anti-Fog.
You spray it on the inside of your visor, but it works on your glasses too. You have to reapply it about once a week, but that's no biggie. Just remember to do BOTH sides of the lenses!!
Option 2 is a *very* small dab of Fairy washing-up liquid. Same as above, wipe it all over the lenses, water it down a touch if you need to. Reapply as and when.
You can also use spit, as you would for a diving mask, but that's a bit gross and doesn't work if you've eaten/drunk anything recently.
I bought a little bottle of Anti-Fog about 7 years ago and it's about half-used, which is rather good given that I ride every single day in all weathers!
Keep it with a small cloth in your pocket or something, as you can simply stop, apply and ride straight away if you ever get caught out.
