An addition to Rorrkonn's list of suggestions (In regard to postwork goodies)...

Corel Painter is more geared to natural media effects than photoshop. A graphics tablet is a must for using Painter, though.

For Photoshop the Alienskin plug-in 'SnapArt' is pretty good for emulating various media from comics techniques to painterly effects.

For more versatile and controllable sketch style effects, the AkvisSketch plug-in for photoshop looks pretty good... haven't gotten it yet but it's not very expensive, definitely on my wishlist.

I have some some really nice Poser Sketch presets that I got from Animotions in their freestuff, made by an individual named ironbear. Not sure if its still available but, very useful.