Wow, that was quick, thanks everyone for the feedback!
My earlier models (including the Fire Warrior pictured) were a bit less clean. I think they've improved some, here's a sneak preview of my Riptide (still needs to be based, I'll upload a full image when it's complete):
But the tips are well received, for sure. Thinning down the white paint isn't something I've tried, but I'm sure it will help make it smoother. On my next batch of models I'll give it a shot.
Highlights aren't something that I've done on my Tau, really. I think probably I should, using white highlights on the blue patches and something a little darker on the edges of the white bits (using grey, maybe, or black with very light drybrushing). Certainly, adding drybrushed highlights worked well for me on my Daemon army. I wasn't planning on posting them up here, since this is a Tau forum, but here's a taste: