Pine is definately not the best to burn on and in fact is dangerous. Buy some inexpensive birch or Italian poplar plywood and use this for practice until you get the hang of it. They are both inexpensive and relatively easy to burn and safe!
Some important things that I usually recommend:
Sand, sand, sand...start with 200 grit, then go to 400, then 600 then 800. I find that the smoother the wood, the better the burning. IMHO sanding is the key to getting a good burning.
touch down and landing is important. If you just touch the pen down on the wood you will get what I call "the dreaded blob". Think of the pen as an airplane coming in for a landing...it's moving as it touches down on the wood. You will not get brown blobs by doing this.
Nedra