I fell in love with this technique in school while we were learning about impressionist photography. I started experimenting with the different effects it produced and made it my own.
Vaseline can give you many different effects. These are just a few I like to use:
Put a small amount of Vaseline around the lens and smudge in a circle pattern, leaving the center clean. This gives the photo a tilt-shift effect:
Dip your finger in the Vaseline and cover the whole lens filter evenly. This makes a glowy effect:
My favourite is putting a glob of it on the lens and smearing it horizontally to get this effect:
You'd think you'd smear it vertically, because the trees are smudging that way, but it's actually the opposite. Plot twist!
Different amounts of Vaseline can also vary the look, and the more you put on the lens, the more abstract it looks.
Is it a photo of a building?
To top it all off, here's a music video I created that features all of these effects: