In reply to Bobling:
I find mine start to mist when they start to get dirty, so give them a good clean in shower gel and then spit in them before a swim.
Failing that, Speedo do an anti-fog liquid.
You don't say what brand you use, but I find Zoggs have the best anti-fog coating to start with - my last pair of Speedo goggles weren't great for that.