Choose a target numeric base:
We start by making a series of 5x7 bitmaps that represent the letters and control codes in your input:
Note that there are five solid characters at the head of the signal, and seven solid characters at the end. This is called the "meter" because it helps set up the rhythm of the 35-bit sections (as well as reinforcing the importance of the prime factors).
The final result is produced by inserting a zero at every 36th bit:
If the recipients of the message are mathematically-minded, they'll notice that 35 is a semiprime number...and thus only divisible by 5 and 7. This should cue them into the idea of trying to decode it as a 5x7 bitmap. See if your friends with geek-cred can figure it out, or send them the decoder if you are feeling generous. :)
(Thanks for stopping by! If you find ideas like this interesting, then you may also be interested in some of my other projects...like blackhighlighter. It seeks to aid transparency in communication by making it easy to publish most of your correspondence in public, while still committing to what you said in the hidden parts. Check it out!)