For many years I used a folding plastic handled Rapala fillet knife.  About 10 years ago I bought a Don Canney Leech Lake knife and I now use this exclusively.  This knife has a 7" blade that is sharpened on the last 5" of the backside.  The reason for this is that you use the back-side for cutting areas that may have knife-dulling bones, and then use the long side for filleting out the meat.  I fillet a lot of perch in the winter so this makes a difference to me.

As for sharpening, I tried all sorts of stones, sticks, and plastic fixed position type things and never really had good success.  Perhaps it's a question of skill, but I could never seem to pick it up.  An uncle of mine in Sweden showed me a Lansky system and now I use it for all my knives.  It was easy to learn and does a very good job.