I suggest maybe having a quick look in the tutorials on this site (Use Aharon's tutorial on creating tight junk mattes as a good starting point
http://www.creativecow.net/articles/rabinowitz_aharon/junk_mattes/index.htm...
..and another good one was an old free video tutorial from Total Training on using vector blur
http://www.creativecow.net/articles/total_training/AE65/Vector_Blur/index.h... I don't do much DV keying work, but I tried that trick a couple of times and it gave a nice looking result with the edges. I also seem to remember Barend doing a bit of a review on keylight when AE first got it with a quick run through, so have a look for that). Basically the setting depend on what the shot it like, a lot of the time its mucking round with a small bit of screen strength, maybe a dash of pre-blur (being DV) and then while looking at the matte, use the clip white and clip black to get a good matte1. If you have problems with edges I suggest using proceedural matting techniques (again Barend did a tutorial on this ages back).
Maybe doing a quick search on DV keying tips and tricks on this forum (especially looking for the ones that use lumanance (green) channel tricks to get good edges (because in DV, green is the least compressed channel, ...if you take your image and look at each channel individually (using the red green blue buttons on the main display) you'll see how nasty the red and blue channels edges are (blocky) compared to the green channel.
Goodluck!!
Cheers,
andrew
