What's the best way to protect a small outdoor tropical tree from frost?

How much you need to protect the tree depends on how delicate the tree is and how bad your frosts get. Some "tropical" trees are actually just fine down to occasional 20 degree temperatures. Other trees will just up and die at anything below freezing.

Things you can do to protect a plant:

+ Water it. A well-watered plant can better protect itself from frost.

+ Cover it. Even a light cover can prevent a lot of damage. Just toss an old sheet over the tree.

+ Buy "cloud cover." its a spray you can get from garden stores that will protect the leaves from damage. You'll have to reapply it after a rain.

Obviously you can't do this all year so if you live in an area that freezes all winter you may just lose the plant. If you only get occasional frosts you'll have to pay attention to the weather. In general dry, still nights are worse than wet or windy nights. I find that if the teperature has dropped below 40 degrees and there's no wind by 9 or 10PM I'd better go out and cover my lemon trees. If its just cold and rainy I don't worry so much.

Good luck!