Frequency Analysis

In large chunks of text, the relative frequency of letters tends to be roughly the same. This gives a potential strategy to break any simple substitution ciphers easily

1. Assign the most frequent unassigned letter of ciphertext to be the most frequent unassigned letter in some sample English text (or perhaps some other letter with a similar frequency).
2. Look through the ciphertext and see if you can make any guesses about words that seem to appear there. If you see something, fill in the blanks in that word by making appropriate guesses for the key.