I have a puncture wound from a rusty nail and can't remember when my last tetanus shot was. It has been very well cleaned. Advice?

Depends. Today it is common for OBGYNs to recommend the TdaP vaccine during pregnancy, but people remember it more for the pertussis part. If you had one with a pregnancy within the past 10 yrs you may be okay, if not the vaccine is a reasonable precaution and would give you protection now & for a while.
Be Immunized. A lot safer to get the shot early than to risk a serious disease.
Tetanus shot. If you can't remember when you got your last tetanus shot, or if you know but it has not been within the last 5 years...Then you need another tetanus shot.
Get treated. You may have been exposed to tetanus and need a booster , the least you have done is possibly inoculated yourself with bacteria and may need antibiotics.
Tetanus Risk. The basic recommendation is to have your tetanus shot updated every 10 years. However, it is recommended that if you have a particularly dirty wound - that you insure you have had a tetanus shot within the last five years. Puncture wounds pose higher risks. If in doubt - get that tetanus shot updated.