there are a number of filters available as well as some "actions" but they will ALWAYS give the piece a digital "filtered" effect.

The best way is by hand, slowly building up tone and value adjusting slightly to a different hue, making new strokes blend with your first set. It's not easy but you have better control and the finishes piece will rival a traditional piece. It helps a GREAT deal if you are already familiar with painting with watercolour. In this way Photoshop is just another tool...

:)
retrocity

P.S. some of the filters i mentioned can be found in the FILTER --> ARTISTIC --> Watercolor...

try experimenting with:

• different settings
• working with multiple layers
• change the "blend mode" on some of the layers

remember if all the layers are aligned, you can erase areas and apply your effect to specific sections giving you more control...