According to Henshall, the etymology of 卓 is pretty obscure. That doesn’t prevent him from writing an interesting half-page long analysis, which I can’t do justice here without quoting it in full (which I won’t - you really should get your own copy of the book).
There’s a funny twist in that Henshall claims that 早 consists of the precursor of 七 (seven) and 日 (sun) while the popular interpretation is that 早 depicts the sun rising above plants - hence ‘morning’ (which would be a great mnemonic, weren’t for the fact that 早 already is really easy to remember). In the case of 卓 it seems somewhat likely that this character did originally consist of a sun rising above a plant, hence the basic meaning ‘rising high’. Unfortunately it’s unclear what the meaning is of ⺊ at the top of 卓.
In Japanese 卓 is also used instead of 棹 for the word ‘table’ which the latter was used for in Chinese. This means that in Japanese we’ve got two classes of words which use 卓 in their spelling: compounds with ‘table’, like 食卓 shokutaku (dinner table) and words like 卓越 takuetsu and 卓抜 takubatsu (which mostly share the meaning of excellence, rising above, etc.). There seem to be more words with table though.
Since there is very little history to connect 卓 with ‘table’ (a bit more for ‘excellence’) and since ⺊ has an entirely unknown function in this character, you might want to play with the elements a bit in your head to help remembering. Kanjidamage suggests linking taku with talk to your family at the table! for the pronunciation. Since 早い is easy, something like be early at the table might work. ⺊ or ト might remind one of 卜 / 占 uranai (fortune-telling, predict) and for 卓越 and 卓抜 predictably rising high/excel might be an association. Weirder still: be early 早い at the table and predictably ト rise above the rest.
