Sadly, I don't have an SLR camera, so this one is kinda impossible for me to do manually. My Samsung PL100 (and accompanying black spot ... if it were a pirate it would be doomed) does not allow me to manually set the aperture of a photo. BUT, I have discovered that by using two different settings in my camera, the "Close-up" and the "Landscape", I can tweak the focus point of a shot. This is a sort of hack for those of us who don't want or can't afford a $600 camera. Yeesh.
My camera has a box or brackets that indicate the focus point of a shot. Yours may or may not have this, too.
Images shot in "Close-up" mode are indicated by a tulip icon on your camera that looks something like this >>>
Images shot in "Landscape" mode have an icon similar to this one >>>
Sooo, here's examples of the two modes on my camera with (mostly) the same shot.
See how the focal point changes? This one way to get a blur going on. You would usually get by changing the aperture settings on an SLR.
Try switching between these modes on your camera and see what happens!
You can also manually create a blur with Picnik. I applied the "Focal Soften" effect to this:
|this one's got a little Vignette going on, too|
Whether natural or filtered, aperture is a great way to give your photos depth.