A mnemonic a teacher at school taught us several years ago: Dashing In ARush, Running Hard, Or Early Accident. You'll never forget how to spell diarrhoea again!
My own Mum's prim version was "Air raid backwards". The opposite was "knobs". It was all "po". (SM) Yes, but what do S-K-W-I-T-S stand for? The SHITS, by the way, is a section of the band of the Irish Guards who call themselves the Society of Harmonious Irish Tunesmiths. This is true.