Poor communication kills. We need to know exactly how many bullets are left, where the nearest shelter is, and how many reanimates there are, not "some", "to the right (No, stage right!)" and "sort of a lot". But you also have to be careful you're not revealing to much to the opposition, lest they use it against you.
I find that when it comes to citizens, the simpler, the better. They still have plenty of computers and cryptographers around - scrambled transmissions and cypher codes are too easy to break. However, they are not very familiar with idioms or slang - so take a page from the Navajo code talkers - substitute words, preferably in a technical or foreign language. I've heard Latin has had some success . I wouldn't trust it however, being the root of other languages, they may be able to work backwards. Hebrew, Hindi, and other non-romance languages are preferable - at least if you're not in that part of the world. (Citizen Outposts adapt to the local environment, just like a good scavenger should!)
More complex codes work against humans, if you can manage it - which you probably can't. Really - you're evacuating from the house because undead are breaking down the door - do you grab an extra gun or the two to the 256 to the 256th bit encoding mechanism? Did you even have one in the first place? Yeah, communication is pretty easy to monitor these days, especially since you just need the right HAM radio channel for most of it. Good couriers are well paid these days, and military communications gear may soon top even farm equipment as the number one in demand item. That is one of the advantages of Vegas - they have the communication thing licked - if only they had outsiders worth talking too!