Cryptography Challenge

Several years ago I developed the idea for a cryptography challenge. I think it was right after I saw the film U-571 and was thinking about the Enigma machines used by the Germans in World War II, and somewhat about the Navajo "windtalkers" used by the Americans, and I just became somewhat interested in effective cryptography and codemaking and codebreaking.
The rules:
1. The code has to be fairly easily memorisable - there has to be some kind of a reason to it. This is because for a code to be effective, in my mind, there needs to be no physical evidence of it - no key, no machine that if a copy of it was stolen could be used to decipher everything.
2. The code has to be repeatable. The idea here is that if 'they' (whoever is trying to break your code) capture a user of the code and are able to force them to produce a particular paragraph in the code, it will always be the same. This isn't really perhaps so tied to a real thing that you'd want in a code, but it has more to do with my sensibilities of reasonability and fairness. This condition developed because one of the first codes I developed was basically that only every fourth character mattered. This made the rest of the code completely unimportant, and then when I wrote it out again for someone to try to crack it (they'd lost the first copy or something), it was of course noticeably different.
3. Codes do NOT have to be cryptograms, i.e. 1-for-1 letter substitution. Those of course are acceptable, but generally basic, especially if they actually follow rule number 1. And this isn't so much a rule as it is a lack of a rule, like USCF rule 20F1.
4. Codes can either be merely for alphabet, alphanumeric, or incorporate symbols and punctuation as well. Perhaps later there should be a rule requiring the telling which of these sets is being encoded for, but we'll start without that. What do the participants in this challenge think?
5. Codes, as far as this contest goes, should not require knowledge of any spoken language other than English.
6. People interested in submitting a code should do so by submitting a sentence (or preferably several sentences) of text in code. This should be long enough, when decoded, that the decoders have some sense that what they are doing is right. And they should be coherent, so nothing like "Bats lick ice cream jumping under the sand to fall on the moon." It might be helpful if there was some standard sentence to be coded, but that would probably be too easy, so I would more suggest a hint, like "This is a Star Wars quote".
7. There are probably other things, but I'm forgetting them right now.
I really don't know how good I am on either end of this. It's my intent that people will submit encoded messages to be cracked and that other people (likely the same groups) will try to break the codes. This is purely for fun, or maybe for developing some codes to actually be used in your day-to-day live. Paranoid as I am, I actually have a code that I've been developing for several years that I use for keeping information in such a way that other people can't easily read it.
I will provide examples if requested, or if I think of a good code that I want to see if you guys can break.