So make sure that you have a collection of guitar picks. If you are low on cash, just keep all the picks left at your studio on a little organizer box. You'll find more picks than loose change in your couches (unfortunately) so make sure you keep them all together. You'll thank yourself later.
The most specific portion of the acoustic guitar tone that picks can have the biggest impact on is the high end click. All acoustic guitars make a clicking sound to some degree when the plastic (or possibly metal) pick attacks the metal guitar strings. However, some picks make this click much more prominent. So much of this depends on the player that you really need to experiment to find out what works best in a given situation.
Picks can also have a big effect on the low end of the guitar as well. Let's go back to my situation with the very thick purple Dunlop Tortex picks. As I said above, I'm just barely grazing over the strings when I strum. Imagine a chick (preferable hot) lightly strumming a harp or something and you won't be far off. Strumming in this fashion typically keeps the volume of the guitar down and keeps the low end of the guitar controlled as well. This sound certainly has it's place, but sometimes I need more low
end in my acoustic guitar sound. By using a thinner pick, I tend to strum harder. This gives the guitar more body. To further complicate things, if you gave one of my thick picks to a guitar player who doesn't back off to account for the thick pick, you'll get a massive low end. So sometimes thick picks can give you a thicker, bass heavy tone and sometimes they can give you a thinner tone. It all depends on the guitar player.