Dominic O'Brian material is similar to Harry Loraynes and Kevin Trudeau.
Robert Finkel in his book Memory Booster has a technique he refers to as the telescopic technique. Based on first orgainising the information, then creating visualisation, and associations OVA.
His technique for memorising a chemical chain would work well for formulas unfortunately it needs the diagrams to explain it (visualisation).
Mind mapping seems irrelevent for formulas however have you drawn a flow chart? The act of drawing it up in a flow chart will probably anchor a good deal of the formula into your memory and if you do it using colour you'll create a visual image you can call into your minds eye.
Alex
