Do It Yourself
The safest and most effective way to get a cat out of a tree is to have it done by an experienced and qualified tree-climbing cat rescuer, but some people just want to do it themselves, and, sometimes, you simply have no choice but to do it yourself. For that reason, I will present some do-it-yourself options that you may want to try. Some of these methods are useful only in a very limited number of cases, some are difficult to execute, and all of them have a far-from-perfect success rate, but when you are able to do it successfully, it's a wonderful feeling.
Safety First
Please keep safety in mind -- not only yours but the cat's as well. If there are any power lines nearby, don't do anything at all except call a professional. Your life is on the line, so don't take any chances. You also need to be sure that you will not disturb any bee hives or animals that may be in the tree. There could be large, broken branches hanging loosely overhead that could fall with little or no disturbance. The tree itself could be unhealthy and on the verge of collapse even though it may appear to be fine.
This is certainly not an exhaustive list of all the hazards you may find; many things can go wrong. When in doubt, call a rescuer instead. With that warning stated, below are some ideas you may want to try. If you have any other creative solutions that work to safely and successfully bring your cat down, I hope you will share them with me.
Do-It-Yourself Rescue Methods