Hi Kitten, I'm with fly. The obvious ones you CAN take for granted i.e. NYC SFO CHI LON and so on. Take the time to remember the oddball ones like MID (Merida) etc. Like any other test you have to take the more you work at it the better you will do. If you can, try to get hold of a copy of the OAG or the European ABC airline timetables. They all have a section with the city/airport codes in them.
Lotsa luck kitt
Heat