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Also, when taking your leave, be sure never to forget it.
Literal
Also, take-leave-do [time-際に] never forget-not [so-as-to-ように] do-thing.
辞去 is a stiff, formal noun for politely taking one's leave (departing from a host or superior), characteristic of written instructions and etiquette manuals. ~する際に is the formal equivalent of ~時に ('at the time of'), preferred in stiff/official language. The ending ~ようにすること is a written instructional pattern: ~ようにする 'try to / make it so that' + the nominalizer こと, letting the whole sentence stand as a directive without a finite verb. 決して…ない is a fixed frame: the adverb 決して 'never' obligatorily takes a negative predicate.