If online, post the letters L, O and L (in that order) to indicate laughter in real life. Optionally the letters X and D may be used to form a so called "emoticon", for added effect.
If (or perhaps when) making a joke yourself, you can also use a (semi-)colon and o or dash and a closing parenthesis at the end of your sentence as a hint to the recipient who may otherwise--in the absence of body language or tone of spoken words--not notice that your utterance was meant to be humorous.
However, in some circles this may be frowned up on as an insult on the readers intelligence.
