Swanny has a good article on puttying seams on his web site at:
http://www.swannysmodels.com/Seams.html
I use a palette spatula I got at an artist's supply store. I wouldn't recommend using your fingers, nor if you use any of the toluene based putties would I recommend breathing the fumes.
There are several threads about puttying seams in here, and one recommendation I saw but haven't tried is for porcelain sealer. Here is a link to that one:
http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=19822
I've also heard ot people using auto body filler, and other non-modeling types of filler.
Squadron putty works pretty well, I use it quite a bit. It will adhere to styrene but not to resin. It is also a bit grainy. If you let it dry thoroughly it polishes pretty smooth but if you try and sand it before it's dry it doesn't finish very smooth.
Milliput is a 2-part epoxy putty. I've only used it once and that was just to add some internal support. I don't think it is really suited to seam repair since it is very stiff and won't form into the seams that well.