I've spent a lot of time backpacking on the beach with fires in the rain. Here's my experience.

Any big clear bag put over you helps. When you stand near the fire (not too near) it heats the inside of the bag and you.

Here's another option that takes a little more work.

(1) Tie a string between two trees so it creates a "clothesline" that runs by the fire about 7 feet off the ground.

(2) Hang a tarp or piece of plastic vertically from the string so it forms a hanging wall.

(3) Stand between the fire and the plastic wall with your back touching the plastic. Now step back (into) the plastic about a foot. When in this position (assuming the wind isn't too bad) you are warm and protected from most of the rain. What little rain might hit you will quicly evaporate.

One nice thing about the hanging wall method is that you can easily come and go to tend the fire, collect firewood, cook, etc. If you get a little wet on these trips you can quickly dry out when you return to the proection of the hanging wall.