Use a "soft" (right word?) brush. One with a smooth edge. And to add a little glow effect, I use orange (if you use red) and bring out some main points (agein, right word?) in the middle of where it should glow.
....
Say if that wasn't understandable, then I give you an example. ^^"Okay, I just made a quick one. It's a bit crappy, but you should see, what I mean.
Of course you have to use a soft brush for the orange shades, too. Mine looks a bit... yeah, crappy.
Um, did you mean that? Does it help you?