Find your inspiration Pinterest is ideal for this. My personal hair-inspo's are Sienna Miller, Ashley Benson and Sara Donaldson. If you're looking to cut your hair or recreate a look, having a ton of pictures will help your hairstylist know what you're going for.
Use heat, but sparingly In an effort to not damage hair, heat products like flatirons and curling irons should be used but not too much. Making flatiron waves (by creating a light s-shape waves through almost crimping the hair) and using a 1-1 1/2 inch curling iron to do the loosest of waves will give your bob the wavy, messy look. I have watched quite a few videos on how to correctly curl short hair and they actually really help! Check them out here, here, here, here and here.
Use the right products Oribe and Bumble and Bumble will become your best friends. Unless you have naturally bedhead hair, a couple of products are necessary to maintain the cool girl bob. The Bumble and Bumble Don't Blow It Creme is really nice because it creates natural smooth waves without having to really do anything. With surf sprays, the Bumble and Bumble Surf Infusion spray is (in my opinion) far superior to the regular Bb surf spay because it doesn't dry hair out as much. The Oribe Apres Beach Wave and Shine Spray is my personal favorite for a couple of reasons: it smells absolutely perfect, it's build able without being heavy, and it truly honestly works without making your hair feel gross-peachy. For texturizing, there's the Bb Texture Creme. I like to put it in to rough up my hair a little. Some fan favorites are the Bb Dryspun Finish and the Oribe Dry Texturizing Spray.