I came across an interesting page today entitled "IM Etiquette". Some of these tips are more important than others, but people who use good etiquette when IMing me (and probably many others) are much more likely to get a good response.
Most important tip:
"Unless it's some sort of joke or prank, don't leave more than two consecutive
messages for someone who you know is away. The same goes for if you're talking
to someone and they don't respond right away. They may be otherwise occupied.
There is no reason for "Hi!", then five seconds later "Hello?", then "Anybody
home?!". This elevates into ridiculousness and you'll only lose the other person's
respect. And they will want to ninja-kick you."