Yesterday's post about lenses that aren't terribly sharp might have made you think of premium soft-focus portrait lenses like the Cooke PS945 or the Rodenstock Imagon. Or, if you're not a large-format kinda person, maybe the Lensbaby or the Holga. As you probably know, there's a whole toy-camera aesthetic out there known by the general name of Lomography, after the website of that name and the Lomo camera. I've never liked diffusion filters—it's just a very false look, to my eye, especially the kind that selectively makes whites all glowy—but maybe you will.
Nothing wrong with playing with any of those options, of course. With many such options, people get into a "now I'm doing soft focus" mindset, and turn it up to 11 (moar cowbell)...they just go way over the top, and make pictures that are grossly smeary or fuzzy. We humans have a great fondness for excess.
Personally, I've always felt that if a viewer notices an effect, then it's been ladled on too heavily. That goes for the oversharpened eyes that Amin was complaining about (sharpening in general, actually); any of the standard tricks for making faces or bodies more appealing in Photoshop, like making the eyes bigger or the legs longer; soft focus; saturation...anything. The viewer should sense the aesthetic impression but not recognize immediately how it was achieved. That's just my opinion.
And to experiment with soft focus, you don't really need to buy anything. As long as you have a UV or clear protective filter for your lens, you can experiment.
Try putting "dots" on the filter with a black Sharpie. You can try even dots all over the filter, or leave the center clear and make the dots more dense toward the edges; then try shooting at different apertures. Don't like what you did? Just rub the dots off (or scrape them off with a single-sided razor blade) and start over again. More dots or fewer, larger or smaller, all creates different effects. You can use a hole punch and black construction paper cut in a ring shape to construct an Imagon-style "spherical" soft focus effect filter, mimicking the one in the picture at right. Too much? Too little? Try again. (Note that whenever you have any soft of center opening or gradation of effects on the filter, different apertures will yield different effects. If you're shooting digital, it's easy enough to try them all.)
Can't you just do it all in Photoshop? Maybe. Sometimes you can, sometimes you can't.
Using old or bad lenses is a useful trick, although you're dependent on the particular lens you find. Christopher Bailey used to use a Nikkormat with a cracked lens and stuck shutter (it only worked on "B") to make his night shots, and was out of luck when his car got broken into and the "special" broken lens got stolen! The irony is that it wasn't worth anything to the thief, even though it was worth a lot to Chris.
Everybody knows the trick of smearing vaseline on the filter (maybe just at the edges) or stretching a nylon stocking over the end of the lens. Again, too heavy-handed for my taste—what you usually create is not a subtle image modification but an EFFECT that shouts at people NOW I'M USING AN EFFECT!!! My advice—worth what you paid for it—is "think subtle." Just shade it one way or another. Tap your viewer on the shoulder rather than bashing them over the head with a brick.
If nylon stocking material is too much, the fabric store is your friend. Find some loose netting to put over your filter. Note that white and black fishnet have different effects on the image. (Colors, too.) See what you can find. Try cutting a small hole in the center of the netting and just using it on the edge, then trying shots at different apertures.
Once you get an effect you like, you can add it to your bag of tricks, and use it when you need it. For the most part, though, it costs very little to experiment. Even if you never use any of the effects again, you'll come out with a better understanding of visual effects and how images work.
ADDENDUM: Oh, I almost forgot: you used to be able to make homemade diffusion filters by spraying hairspray on a plain glass or UV filter. Put the filter on the ground and spray the hairspray into the air and let the tiny droplets drift down on to the filter. Allow to dry, and use. More diffusion, more hairspray. When you're done, just wash off the filter.
This was a tip dating back to the era of beehive hairdos—it was hoary when I was a pup—and I'm not sure hairsprays are still the same kind of thing. Experiment.