Pidjy's suggestion should work in most cases. You might want to add a light, to give the illusion that the glow casts light on it's surroundings. If you want the glow to look more intense, you have to work with a dodge-layer in Photoshop or PSP or you have to use a dodge-brush to intensify the glow and the falloff. You can use all sorts of textures, but most important settings are volume/fuzzy and additive. If a glow object receives no light (diffusion and specularity both at 0) you can also turn all shadow options off to slightly improve rendertime.

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