Most newspapaper have a section with such phrases already encoded on a daily basis, just like crossword puzzles.
I wuld be tempted to increase the complexity too quickly to be much fun, such as deleting all the spaces. Ouch!
Oddly, in that instance, the longer the quotation, the easier it is to decypher.
Another way to mix the alphabet is to arrange it in a five-by-five lattice, (with the Z discarded), then to move at right angles to the order inserted.
ABCDE
FGHIJ
KLMNO
PQRST
UVWYY
Down three makes A = P, B = Q, and M = C.
It can quickly grow horrendously complex.
The recent movie "National Treasure" alluded to long kept secrets. Some may be real.
There are some open-encoded texts, based on choosing letters from known documents in some manner, (documents such as the Declaration of Independence), which have yet to be decrypted, primarily because the reference document is not known. It could be the Constitution, or it could be a book of the Bible.
They've been analyzed for years, without solution.
I read a bit once on cryptography and found it very interesting. But the problem with cryptography, if I remember correctly, is that whatever method is used to create encrypted text, the communication of the decoding key to the recipients is really the crux of the matter. That is why this guy (forgot hos name) at MIT developed the PGP algorithm with public keys and private keys. The plain text is encrypted with the private key and decripted with the public key, I think.
I think Zimmerman is his name?
The write-once method is the safest and the one most widely used in intelligence.
Forgot most of this stuff.