As I've mentioned previously on this blog, the "MUDPALES" mnemonic for anion gap metabolic acidosis is one of the most successful medical mnemonic's of all time.
A less successful (and admittedly less useful) mnemonic exists for non-anion gap metabolic acidoses (NAGMA), which I learned as a resident. It's "HARDUP", which stands for the following H = hyperalimentation (e.g., starting TPN). A = acetazolamide use. R = renal tubular acidosis (Type I = distal; Type II = proximal; Type IV = hyporeninemic hypoaldosteronism. D = diarrhea U = uretosigmoid fistula (because the colon will waste bicarbonate). P = pancreatic fistula (because of alkali loss--the pancreas secretes a bicarbonate-rich fluid).
Practically speaking however, the two main causes you really have to remember for NAGMA are DIARRHEA or RENAL TUBULAR ACIDOSIS, which 90% of the time you can distinguish between based on the history alone. Another way to think about the differential diagnosis of NAGMA is to ask whether or not there is GI LOSS or RENAL LOSS of bicarbonate. If the history does not provide an obvious explanation, one can distinguish between GI versus renal bicarbonate losses by determining the urine anion gap (urine AG = urine Na + urine K - urine Cl), where a positive value indicates renal bicarbonate loss whereas a largely negative value indicates extra-renal bicarbonate loss.