I have a pretty good stash of filters for creating soft, diffuse looks, both Cokin, some Softars, and some homemade "hairspray" filters. There are a number of different looks. White tulle fabric makes a good diffusion material; pantyhose material will also work. BLACK-colored tulle, or other black netting makes a really nice diffusion look. Tiffen's Black Net filter is pretty good. Cokin has soft-focus, fog, and diffusion filters, and those work pretty nice on longer lenses, like 70-200 or 70-300 zooms.
Hairspray, sprayed into the air and allowed to fall down onto a UV filter can make a nice diffuser, and it's not messy like Vaseline can be. I have several of those. They work pretty well. THe effects of front-mounted filters depend on focal length, and also how close to the front of the lens they are, and also the f/stop. Black paint also works great" the KEY is to do this spraying from a spray can at above 76 degrees, so the paint will FULLY aerosolize, and it will then drop to the filter about 18"-2 feet below, and form nice micro-droplets as it drops; I also do this for my custom lure painting hobby. You need the air temp, and the paint to be 76 degrees or above.
In photoshop I often create a duplicate layer, I Gaussian blur that anywhere from 45 to 75 pixels (yes, 45 to 75!), then use the Multiply blending mode most of the time, then use the Curves tool to decrease the blur, and to control how blurring things are. Using Screen as the blending mode keeps things bright, light, and high-key. If you use the Multiply mode, it creates a weird, dark look, kind of like diffusion under the enlarging lens, which usually looks odd, but which can work at times.