Google "Johnny Grip". It was suggested to me a while back from someone else on here but only works on wet tile so it's exactly what you're looking for. Let us all know how it works out for you!

The anti-slip treatment arrived three days ago and just my luck it was raining so I had to wait till the day before yesterday to apply it. It then happened to rain yesterday so I was excited to get up this morning and check out the patio. Thanks for the suggestion babypops – the tiles have way more traction underfoot now, even when they still had small pools of rain water on them!