Formal:
* Rarely: This is a very common and straightforward way to express that something happens infrequently.
* Seldom: This is similar to "rarely" and implies that something happens only occasionally.
* Infrequently: This emphasizes the low frequency of occurrence.
Informal:
* Hardly ever: This is a more colloquial way of saying "almost never."
* Never really: This implies that something happens so rarely that it's almost nonexistent.
* Once in a blue moon: This is a common idiom meaning "very rarely."
Other options:
* Sporadically: This emphasizes the irregular nature of the occurrence.
* Occasionally: This implies that something happens from time to time, but not often.
The best word to use will depend on the specific context and the desired tone.