My workflow on AO renders.

[paoloverona]

sometimes I use the threshold level (applied to a duplicated layer of the render) to bring out all the white parts in the image, then I apply a blur effect to it....the fusion option for this level is set to linear dodge...that's all

This is the only method that I know

[PAQUITO]

Ah Ok. What I do is select>color range, and take a sample in the brightest spot of the image. Then with the fuziness control you decide how much the selection spreads. In a new layer, fill the selection with a solid white, deselect and apply a good blur. Then with the opacity of the layer you can control how strong you want the bloom.