Traditionally thought of as a lazy-day activity for firefighters, rescuing a house cat that somehow winds up trapped in a tall tree presents quite a task for the uninitiated. By nature, cats are timid, instinctual creatures who quite intelligently treat anything unfamiliar as a threat.
Typically a cat that is stuck high up in a tree is in a frightened state, and perhaps will find its own way down after some time has passed and it no longer senses any danger from the immediate environs. As such, the best 'first step' is to wait to see if the cat climbs down on its own.
In many cases, the cat will find itself too frightened to move after a few hours have passed, or may begin meowing continually -- signaling that it might be trapped and is calling out for assistance in getting down. Two items that are useful in this case would be a ladder and cat food. Using the cat food first, try opening the can and leaving it underneath the tree to coax the cat down on its own power.
If the cat refuses to come down even then, using a ladder is probably the best next solution to help the cat down on its own power. Place the ladder against the tree near the cat's location, and allow it to climb down by itself -- climbing up to rescue a cat most likely would just frighten it more, possibly prompting the cat to climb even further up into the tree.
If the tree itself is not that high, and the cat simply refuses to climb down the ladder, it is recommended for a would-be cat rescuer to put on worker's gloves and heavy clothing before he or she attempts to climb up a ladder.
The cat will most likely resist the rescue attempt, and lash out at anybody that climbs up after it. Try grabbing the cat by the nape of the neck, and bringing it down, after reassuring and calming the cat (to avoid any high-altitude accidents on the way back down!).
If none of these steps help at all, the last option is to call a humane society or animal shelter (do not call the local fire station!). These organizations will be able to help rescue the cat quickly and safely using trained professional animal handlers.
Just be aware that this option may come with an incident report and possibly a fee (especially for repeated instances). Humane societies will most likely attempt to take custody of any cat they find has been abused in these situations as well, and may even file animal abuse charges against the owner in extreme cases.