1. Cutting them up into smaller pieces and turning them into mulch or firewood, OR putting the small pieces into your yardwaste bins for pickup.
2. Recycling! The tree can be scheduled for recycling pickup, or you can take the tree yourself to your local recycling place.
In our particular case, the community that my friend lives in held a charity event collecting trees to be put in a wood chipper. It was held at a local recreation center's parking lot. Anyone can bring their trees to them for a small donation fee. This is how we transported my friend's Christmas tree to the collectors...
You can't get any more 'green' than this!!
We certainly got our share of thumbsups from passing motorists and we even got an applause when we rode in to the parking lot where the trees were being collected. Re-Cycling !!