Try a butchers steel first... provided the edge isn't too far gone it'll pick it up a treat. Use minimal pressure on the steel to avoid damaging the edge.
If it's too far gone for a steel, you can use a stone or a sharpening system (lansky, DC3/4, sharpmaker etc) but again, do it carefully and gently - for very thin knives there aren't really any shortcuts other than not letting them get blunted in the first place - I usually steel mine before every use to keep it in trim.
I recently had a go with one of Mr. Longstrider's strops and some smurf poo... not on a filleting knife but on my Opinel, another thin and delicate edge, which it brought up an absolute treat - definitely worth the investment IMHO.