It may be useful to you to have an extra letter at times or to have one fewer at others. Try to keep the use of extraneous words to a minimum. Remember that a hyphenated word can be used for the expansion of either one or two letters. If stuck for one particular letter, try amending one or two others you already have words for. Changing the meaning you're aiming for can assist in matching the stray letter.
Long words are easier to expand than short ones.
They give more scope for word play. Bear in mind that English may not be the primary language of your audience. Don't rush into anger or laughter at what 'you' may be misinterpreting. Don't wait until you need one to create an acronym. Collect or create them in advance.