Sitch:

So you're supposed to be a dedicated friend and listen to their conversation. But how can you stop thinking about that D you got on your math test that you studied so hard (or not so hard...) for? 

And besides, your friend tends to be a blabbermouth anyway, going on and on and on... 

But that's no excuse. The truth is, you need something to say and you need it fast. 

And you need your friend to think you're a good friend. Soo... 

 Tips:

1. Study the person's expression. If their eyes are wide and hopeful, even expectant, and their mouth closed or pursed, then they asked you a questions and are waiting for you to respond. If they're cracking up, they're probably hoping you'll laugh along with them. If they look slightly sad, they just babbled all about their latest problem. Etc., etc. You can probably figure it out from there.

2. Use a general answer that could be used for just about anything if you can't read their expression and are hopelessly lost.

More:

If you THINK they asked a question:

"Mm-hmm." "Maybe..." "Ehh..." "Naaw..." "I'm not sure..?" "Probably." "Well... I kind of don't know..." Or just plain "Well..." "Hmm." Ellipses (those three dots in a row) work really well - as if you're trailing off into an aura or mystery. Or, you know, not paying attention, but, same thing...

If you THINK they were just gossiping about some person:

"Really?" "Are you serious?" "Omigosh." "Whoa." "Where'd that come from?" "I get it..." "I see." "Uh-huh." Or just roll your eyes.

If the person is laughing:

Laugh along with them. Clearly! Don't roar and screech and hold your belly - ya might get some strange stares there (not that there's anything wrong with that!). Just chuckle lightly, like Santa Claus ~ Ho Ho Ho!

If the person is biting their lip or looking generally unhappy:

Sigh and pat their shoulder. Chew on your lip. Show sympathy with your eyes. Lean against your hand - no words are extremely necessary. If you MUST say something, "That stinks..." will work. Keep your words soft and sounding like you're regretting something. "Awww..." "Oh, whoa..." "Geez." Again, work those ellipses.

If the person looks like they said something serious that you SHOULD have been paying attention to:

Widen your eyes until they are nearly popping out of your head and gasp. "Oh!" will work. A simple interjection smooths your earth-to-person mishap... Just remember to pay attention!

If you are completely lost and have no idea:

As said above, use a general, generic answer. "Oh." "Uhhh... Um. Err..." "Well..." "You see..." (pause) "Dunno..."

"Dunno" is a good one because it could be in response to a question, or, if you're voices gets softer and trails off at the end, it could just be a simple comment. Perfect! But if someone is confessing that their boyfriend broke up with them, "Dunno" just won't cut it, if you get the drift.

Or you could just resort to

"Oh, wow, I'm sorry. Spacing out like usual. What was that you said again?"

Probably not a big deal. Your friend, exasperated, will sigh, shake her head if this happens often, maybe throw in a little lecture... Then resume. It won't be the end of the world.

Now, wait - you weren't spacing out on this article, were you? Do you need me to repeat all this?