If you do use a space heater, be careful and monitor its use. You do not want to cause a fire or melt-down of the pipes.

You could use a heat tape, but check its temperature warming rating.

E-plumbers suggestion about the antifreeze is a good one for the trap...use the stuff used in RVs
You do not need to pour alot as only the trap will retain the antifreeze.

: : Bathroom sink and shower drains froze in new bathroom addition. Plumber able to defrost sink but shower still frozen for 5 days. Pipes were insulated by contractor. What can I do to unclog shower drain and is there anything I can do to make sure this doesn't happen again. This bathroom is over an unheated crawlspace on the side of our house.

: Reply:
: Install a space heater in the crawlspace and allow the heat to rise and speed up the thawing process. In order to prevent it, you may need to install permanent heating in the crawlspace or a temp fix would be to pour anti-freeze into the shower drain trap after each shower to prevent it from freezing. Your lucky your water lines didn't freeze.
: Good Luck. e-plumber