How to grill portabello mushrooms properly
I have found marinating in some sort of olive oil/vinegar type of liquid for about an hour ahead works well.  They feel a little soggy/slippery when you put them on the grill but firm up right away and taste pretty good.

Key is to make sure your fire is hot and direct (don't "smoke" them) - I'd say 350 degrees or so - and pause a moment to ensure the grate is hot as well.  Then I would take them off as soon as you start seeing the grill marks on both sides.  Like 1 minute per side. I personally use equal parts Olive Oil and Red Balsamic.  Sometimes I have also used Worcestershire and it makes them taste even more like a "burger".