I once saw my dad help a tree to re-root itself by doing something like covering its end with a plastic baggy or something of the sort. It was quite a while ago, and I don't remeber very well, but I would really appreciate some help on this, because, well, we paid a lot of money for our douglas fir, and I hate to see it go to waste, so I'd like to replant it. Please, and suggestions?

This site indicates that rooting a cut tree is next to impossible.

If you scroll down there is info about rooting Douglas Firs but I don't know that it will work since the tree is already cut. The link takes you to an ad for a rooting hormone but I would think you could use any brand from a garden center if you decide to experiment with it.
http://www.rooting-hormones.com/Christmastree.htm

I've never seen this done...good luck... maybe it was magic

Unfortunately you can not re-root a cut fir tree. Once they are cut they are dying and losing their moisture. Sorry.

Once it is cut it will die..If it had roots attached you could plant it.

Wow I have never heard of that actually working. I would go to a local garden center to find out exactly how to do it. If they don't know or you are too lazy to do that, I would maybe try a baggie filled with vermiculite and water with a little bit of ferilitizer in it. If roots would ever come out, it would probably be at an area that branches were growing out of so I would cut off the lowest level of branches, make a fresh cut on the trunk right underneath this point, and stick the bag with water and vermiculite on. I can't imagine it working, but give it a try.

Wow, I think it is soooo cool you want to do that I wish I knew how to help you.