another vote for the "shoot in raw+jpeg" approach - gives you instant feedback as to if the tones work in B&W - it can take a while to get an eye for what actually works as a B&W shot - one way to train your eye for the shot is to carry a orange filter around (the type old skool film people like me use on B&W film shots) and use it like a monocle... the strong orange filter will reduce the colour levels to a mono appearance (albeit black and orange!). Plus, orange filtering brings out contrasts between cloud/sky and for landscapes, is probably one of the more succesful ways of bringing out contrast in a converted shot.
Edit: don't put it on the lens though - you can do the filtering in Photoshop afterwards, and have versatility - if you orange filter it on camera, you're stuck with what you did.