Scott Kurtz has put out a call for Cintiq knowledge, and I thought I'd share here what I've learned since November.

NIBS:
Felt tip nibs are my favorite. The felt tip ones get mushy, but they won't hurt your screen. They have a nice pencil/Sharpie on paper feel. The plain plastic ones are good for if you run out of felt. There are some ones that are "brush"-like, but they're slippery. Hate 'em, but that's me.

PRESSURE:
Dunno how hard you're pressing, but if you want a heavier line weight, you can put more pressure on your screen (won't hurt it), or make your brush bigger and use lighter pressure.
Try inking with the pencil at 300DPI (at least, you can go as high as 600DPI in B&W without trouble), if you're not. Sounds counterintuitive, but once it's reduced, it looks smoother than the brush.


COMFORT:
A Smudge-Guard! $14.95 and shipping, fast delivery. (http://www.smudgeguard.com/) Helps so much. You know by now the Cintiq gets toasty at the top. This takes away some of the heat, and helps your hand move across the surface. Good for regular ol' paper and pencil drawing, too!
I cut a sleeve off an old long-sleeve shirt and sewed it to fit a little more snug to stay up. This does what the Smudge Guard does, but for up to your elbow. Because the Cintiq can get kinda warm.

OW, IT'S KINDA WARM:
A little desktop fan to blow on underside of the Cintiq. Because it gets freaking warm at the top.

KEYBOARD:
A Nostromo Speedpad. (Amazon.com) Found out about this at the Panel and Pixel Forum. Takes a while to program, and a little more to get used to, but it is TOTALLY worth getting rid of the strain on your keyboard hand since everything you'll need can be put under your LEFT  hand with no reaches for combinations.  Pretty soon you won't even need to look at the Speedpad. Sweeeet.

SCOTT MCCLOUD'S $5. WACOM PEN "TIP":
Program the side switch on the pen to undo and redo.  BEST TIP EVER.

BUTTONS:
I disable the right-hand buttons so my arm brushing across them doesn't  cause anything to happen. I actually hardly use the tablet buttons at all. The touch strip I use for zooming, but that's about it.

TOOLS:
Supposedly you can use French curves and rulers on the surface. I have. But tape some heavy tracing paper or the like over it. Avoid metal.

CARRYING CASE:
Heart-stoppingly expensive. $500. from Digital Graphic Resources. DGR is in Georgia, and they can take a while to ship.