There's always the singular "they," which is my default pronoun for people whose gender I don't know or who identify as genderless.

Yeah, the etiquette is a little complicated, isn't it? Haha.

Really it's not so much whether you slip up and use the wrong pronoun as whether you correct yourself afterward. My partner is much more likely to forgive a wayward "she" if it's immediately corrected than if the pronoun-slipper-upper just blithely continues on.