So, my pipes froze in my house today, and it made me realize that I should better insulate them for the rare days that it gets in the teens here... What should I do to insulate them?
Something like this foam pipe insulation should do the job:
Source - UK site, but you should be able to source this in the States.
It's split down the length so you can wrap it round existing pipes. It's flexible and easily cut so you can do corners and bends too. You might need to tape it a regular intervals to keep the insulation in place if there's not a lot of room between the pipes and any walls/beams.
As well as straight lengths of pipe insulation, you can also get insulated pipe wrap, which may be easier or harder to use in some situations. one advantage is that it can be used to wrap multiple (same temperature) pipes at the same time.
If the pipes are not inside of walls, like in an unfinished basement, one option is to use wrap-on pipe heating cables. They're a bit like an extension cord that you wrap around a pipe and plugin. It radiates. It can have a thermostat to turn on at a certain temp.