I've approached this issue from a cryptographic angle in the past, and came up with a variety of systems of varying levels of security. The simplest one to use works on several basic principles;

1. Each letter (A-Z) is assigned various numeric values running from 1-26, 27-52, 53-78, etc.
2. The first letter of the first word is substituted for the first numeric value it possesses; H for example is replaced with 8.
3. The second letter of the first word is substituted with the sum of this number and and the next number in the sequence; E for example is replaced with 39.
4. This pattern is continued until the end of the word, always using a numeric value larger than the previous one; L for example is replaced with 103.
5. You revert to the lowest possible numerical value with each new word.

HELLO WORLD is written as 8.39.103.219.442 23.64.134.276.566

Added security can be created by making rules, known only to the two parties communicating, that alter the numerical sequence. For example, any number that features multiples of the same digit can be eliminated from the numerical sequence. Without knowledge of the rule the numerical system cannot be translated. Under this rule;

HELLO is written as 8.42.112.254.539

You should have all the information you need to work out how WORLD is spelt.