How long have you got? The short answer to your question is that the callsigns are (in theory) random.
The slightly longer answer is that this is being driven by callsign confusion; confusion between individual aircraft and ATC, between aircraft within the same company, and between aircraft belonging to different companies. The traditional method - using an aircraft's flight number as its callsign - is struggling to keep up with the growth of air traffic, and is arguably no longer fit for purpose. Take out everything that could be confused with a heading, flight level or pressure setting, and the pot of available numbers is already reduced. Then consider that many flight numbers on high frequency city pairs start off with xxx1 on the day's first flight and continue sequentially through to xxx9 or xx10, and that more than one airline employs this method, and you will appreciate the potential for similar numbers to keep cropping up at similar times.
And be in no doubt it is a flight safety hazard. Callsign confusion results in runway incursions, alt busts, aircraft turning the wrong way, you name it. So what to do? Adding letters to the callsign based on route is a good start, but there is still confusion for opposite direction traffic with similar callsigns, e.g. Low Cost 952J inbound on the same frequency as the 953J on the way out (been there, heard it). And, unless it is done in co-operation between airlines, there is still the risk of different airlines with very similar callsigns in the same piece of airspace at the same time. Unbelievably, Lufthansa had a flight with an identical alpha-numeric callsign to my airline, and both were scheduled to arrive at the same airport within minutes of each other. What are the chances?
I'm glad to say that my lot take the issue extremely seriously, and are exceptionally quick to respond and change callsigns as soon as they're advised of any possible confusion, but a permanent solution requires a Europe-wide effort with all parties co-operating. Eurocontrol are working on this, but given the complexity of the task and the number of airlines involved, I wouldn't hold your breath.
As a start, NATS in the UK have rolled out a computer system for use by UK carriers, which is capable of 'deconflicting' callsigns so that no two aircraft should be in the same piece of airspace at the same time with similar callsigns. Several airlines are now using the system, hence some of the tongue twisters (and I bet some of our friends at Swanick just love us at the moment) but, touch wood, I have yet to encounter any callsign confusion since we started using it.