How can I be sure to get a "good" preset white balance reading?
The only thing I know to make sure is that the card fills the frame.
Many times I get the "no good" indication when I shoot the gray card, both inside and outdoors.
How can be sure to get the "good" indication right away?
I have had that problem when I don't have the exposure set correctly before try to do the white balance preset.
I have a D300 and never had problems getting preset white balance work straight away on any decent light. I always get the WB reading with auto exposure, manual (de)focus, and the reference target filling the frame. However, when the light is way off white, I get "no good". This a limitation of the camera: if the light spectrum is extremely unbalanced (say candle light, with too much red and almost no blue), the camera would have to apply a very strong correction (hugely amplify the blue channel) with warrantied bad outcome (too much blue noise).
In this kind of situation, I switch to manual Kelvin setting and I set the color temperature to the minimum (2500 K on the D300). This gives an undercorrected picture with a usually natural-looking warm cast.
BTW, here is a cool WB trick for when you want to keep a slight warm cast under artificial light: Set WB to Kelvin mode, switch on live view, and manually adjust the color temperature to your taste.
On my D70s this used to happen to me until I realized that you need to include other colors/darks/etc. beside the card itself. In other words, take a step back from the white object/paper/card or zoom out and fire again. Try it.
As Nick notes, if you're shooting raw it's not necessary to set the white balance in camera on-site. However, you might find it useful to still shoot the gray card so you can use it as a reference in post (any worthwhile software will have a white balance picker).
One thing to make sure if is that the lighting can vary across the scene, so if you do this have the subject, not you, hold the white card.
I've occasionally run into this when using a high ISO (3200 or 6400). Presumably the noise from such a high ISO is causing the trouble. I turn the ISO down, shoot the reference card to get a reading, then turn it back up to shoot. There's always some color shift at high ISOs so this isn't ideal, but it can be a workable solution.
