The Gemara in Horayos 13 has a long list of things that are good and bad for memory (see Artscroll footnote in particular).
But – To eat olives or to cut your fingernails in order is a lot less of a sin than to sit idly and “kill time” when one has the ability and the opportunity to engage in the most glorious form of avodas Hashem and kiddush Hashem known to man, which is learning the Torah, whereby (in the opinion of the Gaon of Vilna) one fulfills a separate mitzvas asei d’oraysa with every word that one utters in the course of his study.