I've not had much experience of using acrylics but mostly use water colour. Some of the essential things to remember about clouds is that they are usually horizontal in form & I find that using a flat wide brush accross the page helps.
Clouds are usually darker at the bottom edge & lightest closest to the top (light source usually the sun of course, lol!).
For tecniques & help wouldn't an oil painting CD, book or class give suitable advice? Hope this helps.