Um, well my way of doing it uses a bunch of layers, so bear with me. xD
I get my render and duplicate it a couple of times. But get only one layer to be visable, take that layer and go to Filters-->Blur-->Gaussian blur (set the blur to 10)
Now get another layer of the render and put it on the one that was blurred.... If it looks good, then keep it like that, and ur done~
If it's still pasted looking then take that render and smudge the edges of the render.. Then get another layer of the render and put it on top of that, now take the render layer that u just smudged and set it to screen,multiply, whatever looks good, and sometimes u might want to change the opacity just a bit...
And if that still doesn't work you can take topmost render layer and blur the edges a bit, not a lot, just a bit. Oh and u might want to blur the bg where the render and it meet. So that it looks like the bg and render are together. :3
And that's what I do to my renders. Sometimes I leave out a few steps, sometimes I do some more gaussian blur layers at settings of 10+, it all depends on the render and bg, but that's how I usually do it! Hope it helps~ xD (and hope it makes sense. LMAO)
(btw this is from GIMP, so if u use ps it's probably the same aspect, just a bit different...)