if you download the picasa link that Paul posted , theres a good little b&w conversion on there, this is what I do when using picasa to convert
select your pic, then click filtered b&w
move your pointer over the colour grid , you will notice that your pic changes with each colour you scroll over.... reds work best on pics with lots of fluffy white clouds, they stand out
the result from using a red filter
the result using the green filter
you will notice the subtle difference , but its worth playing with all the colours, untill you get a result you l