So far this year, I've probably spent $40 in charges I could have
avoided by terminating instances when I stopped needing them. Often,
I know when I start an instance that it is only to test a short
process and I don't expect to have it running more than the first
hour.

Here's a trick which can be used to start an instance and have that
instance shut itself down in 50 minutes:

ec2-run-instances AMIID --key KEYPAIR --user-data '#!bin/sh
shutdown -h +50 >/dev/null &'

Last night I left 14 small instances running while I slept. Will be running this command first from now on.