I've found that no matter how carefully a model is handled, or how carefully and endlessly it's sanded, that it isn't until the covering iron is heating that several dents appear. If you have this problem also, try this first:

Wet the dent. I usually use spit on a fingertip.
Let it soak in for a couple of minutes.
Gently iron the dent with the iron set hot enough to sizzle a bit on contact with the moist balsa. Iron the wood dry.

I was amazed at how well this works! Sometimes the wood grain rises a bit, so then a light sanding finishes the repair.
I hope you enjoy using this technique! [8D]