Not too difficult
First off body count codes, these are pretty important, so make em easy to remember, we use half of our team name: Team # to mean one thing Rocket # to mean the other, it seems to be a pretty popular way of doing it, a bit more difficult to pick up than "Up 3, down 4" or whatever, but less obvious what is meant.
Other than that the DUPS have used the patent "Steal any code that seems to work" method.
Decide what you need codes for (not that many things!) and then choose words that are easy to shout/hear/distinguish from other ones. I prefer two sillable words for shoutability, I think that this makes them long enough to register that somebody is shouting and then hear what they want to say; but I may be alone in this.
Something important is trust, get to the point where you can call the code and then do what-ever you want to do without having to make sure everyone realises what you mean, that pause can let the opposition know that you are about to attempt something risky.
Have them written down somewhere so all the team can read them and learn them. USE THEM If noone uses them, everyone will forget them, and then your cry of "Bottleneck" for coverign fire on the back right Coke can while you mug the T-bag will just earn you some cries of "What?" and a load of bruises!
There are a load of more experienced 'ballers on here, so if everyone says that that is all b*******s ignore it, but that is my experience.
Hope it is of some help
Richard