While on the subject of DOF (Depth of Filed) and blurry backgrounds, I pause to mention that if all else fails, you can sometimes accomplish the effect in post (post processing). Sometimes, you are just shooting quick snapshots but see something that you like and wish to enhance it a bit. Such is the case with this shot from Danica's birthday party last April. The background is a little too in-focus for my taste.
In Photoshop, I used the field blur option from the blur gallery. I set a blurry point on both the left and right edges. That blurred the whole photo, so to counteract that, I set a number of non-blur points around the subjects. I also used the mask created by the filter to darken the background a little.
This is the result.
Once again, just because I have it for another project, here is a mono version of the same photo.