Choose a pot with a lip that curves out at the top. Turn the pot upside down and smear vaseline all the way around this lip. Sprinkle salt over the vaseline. Turn the pot right way up and plant up as usual (but beware of getting greasy hands if you pick it up by holding the lip!).
The salt does not wash away because the lip protects it from rain. This will last for up to a year, and is very cheap, easy and effective.