I learned about this trick from someone many years ago; and it allows you to get a nice blur effect on your photography. Take any filter (in my case I am just using clean UV 0) and apply light layer of regular vaseline on a glass. The light coming through this layer will distort and reflect on the photo with random artifacts and bluriness.
I've seen some people being very creative with this technicque. You can apply vaseline on the edges of the filter all around it or just on a certain part of it. For example you can use cotton bud to "draw" lines in the shape of a letter "X" (or add even more lines to have a star shape "✶") and while shooting at night the souces of ligh with have that "star effect". Anyways, easier to try and expore yourself.
Couple of weekends ago I was passing by Central Park in New York City and deicded to walk in and take some photos with described distortion. I was using my 50mm f/1.4 lense on Canon 5D Mark III.
Click on the photos to enlarge: