I'll just paste in my response to another query here, in case it is of use to someone.
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I would give superpan and/or sfx a try. A slight yellow filter might give you what you want as well, as might a UV filter or polarizer, but generally I like to avoid putting anything over the lens!
There are many strategies for making clouds 'pop'.... some people simply overexpose the sky and then bleach back the print (with a nonproportional bleach); some selectively bleach, these strategies are sometimes referred to as 'brilliancing' the highlights. Some underexpose and burn, etc. and that can get quite involved like contrast masking. Believe it or not, you can also use the heat of your fingers, during dev of the print, to encourage darker tones in selective spots (just don't try it with amidol, okay?!). This is an ooold problem... but it's been less of a problem in recent years because of the extended pan films.
And there are as many filter suggestions as there are photographers. But the simplest thing, IMHO, is to wield an extended pan film like superpan or sfx... and just shoot it normally or maybe with a mild yellow. Know also that where the exposure goes on the film and on the paper curves can determine how much separation you have in the sky.