For a 5 gallon batch I use 2 oz in an APA/XPA, and 3-4 oz in an IPA. I noticed a huge difference when I went from 1 to 2 ounces, but I haven't done enough 3-4oz batches to decide whether that gives significantly more hop aroma than 2 oz.
I'll use either whole cone or pellet hops, depending on what I have on hand. I haven't noticed a difference between the two, so I prefer pellets when I can since they soak up less beer.
I don't bag my hops, I just dump them in the fermenter (I prefer buckets to carboys for dry hopped beers for this reason). I line my bottling bucket with a paint strainer bag when I rack over to catch any hop bits that may make it through the autosiphon. I have "punched down" the floating hops when using a large amount of whole cones for dry-hopping, but now I just give them a couple of extra days. They seem to take a while to hydrate completely, so I just give them some extra time.
I dry hop for 10-14 days at 68F. I have yet to notice any vegetal character in my dry-hopped beers. I can't say for sure that the warmer temperature makes the difference, but this has been working for me, so I'm sticking by it.