The paper bag and apple works very well.

A paper bag has always worked for me.

If they are not ripe enough for guacamole, maybe they're ripe enough for avocado fries.
http://find.myrecipes.com/recipes/recipefinder.dyn?action=displayRecipe&recipe_id=10000001886172

I've tried the microwave method and it seemed to take away a lot of the flavor.

The window sill or counter works the best. Don't put them in a bowl. If you leave them near a banana, citrus or in a bowl they turn mushy and black before they really ripen properly.

i've heard of the microwave method from the produce man at our market. Never tried it though. I usually do the brown bag/banana trick.

I agree with the brown bag and apple or banana method. Otherwise you are SOL. They take their own time, an under or over ripe avocado is really tragic. If you do use underripe, or that microwave method (new to me), you might like to add a little olive oil to the guac to help mimic a ripe lucious one.

If you have a couple of days, you can put them in a brown paper bag with a ripe apple or banana and leave on your window sill or counter-- if you don't have that much time, I read that you can prick the avocado and microwave for 30 seconds but apparently the flesh won't taste quite as delicious as if you left it to ripen naturally (I haven't tried this method) . . .