You might try putting a glass globe inside the fire, then putting a directional light just under the globe tilted slightly toward the camera, fairly bright and colored harmoniously so that it makes the glass glow inside the fire? I think everyone has the same problem, but that's the best answer I've been able to come up with so far. Generally, with something like that, I'll render the scene, then take it into photoshop and use Eye Candy's Glow filter to boost the fire. You have to do a lot of selecting with the magic wand, but it does the trick usually, and if you choose carefully and then fade the filter effect, the eye candy smoke filter can add smoke to it as well.

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