My advice is: don't!

Usually you will encounter no problems but they you can't take it for granted.
A couple of years ago I was flying in in light rain after putting my Tx in a plastic bag. When taxiing out for a flight the radio simply ceased operating. I couldn't see any water on the Tx but apparently some moisture had found its way into the radio, causing it to malfunction. After leaving the TX inside my house for a couple of hours everything was back to normal.
After that incident I avoid flying in the rain.

/Red B.