Most cats who go up a tree will find their own way down when they're good and ready. But if you've waited a while and she isn't budging, there are some tricks to get her back down to safety.
If You Do the Rescue
Be calm, especially when trying to cajole your cat down. If she detects your anxiety, she might become anxious or frightened and panic, which could lead to her either losing her footing or heading farther up into the tree.
Determine if your cat is free to move about or if she is tangled in a leash, branches or some other material. Clear the yard of any dogs or other distractions--they may be why she took refuge in the tree.
If you don't have a ladder or the tree isn't stable enough for you to climb, try leaving a bowl of food at the foot of the tree. This may entice her to find her way down.
If you have a ladder, lean it securely against a nearby solid object (the tree, a fence) and step up slowly, soothingly calling your kitty's name. Watch her reaction. If she begins to panic, stop your ascent. You may need to come back later. Leave the ladder; it may help if she gets used to see it there.
If you get close enough, coax your cat until you are near enough to grab her. Grasp her by the nape of the neck, then get as firm a hold as you can under her torso to make her feel secure, and hold her close as you descend (considering you will likely be using one hand to hold onto the ladder). Consider wearing heavy garden gloves to protect yourself from swinging claws---or bring a pillow case to transport her back down to the ground.
If your cat truly seems unable to find her way down, call your local veterinary clinic, animal shelter or pet store. They may be able to recommend a professional rescuer or arborist who can help. Professional help may be particularly necessary if it's a kitten that doesn't have the strength or weight to withstand a strong wind, or if your cat is declawed (although more difficult, rear-declawed cats are still capable of climbing trees). If the cat is on an electrical pole, call the utility company. Unfortunately, fire and police departments usually don't have the manpower to handle a call like this.
A cat's sharply curved nails make it easy for them to hook their claws into something (such as your sofa, curtains or leg) and climb upward. But the cat would have to face the same way in order to descend---that is, backward, or butt-first. Going headfirst would be akin to sliding down on an ice skate blade. Cats are usually not inclined to climb or jump when they can't see their destination.
Many people say that cats will come down on their own if they're hungry or tired enough, or that if they found a way up, they'll find a way down. This isn't always true. As with people, cats can become frightened, disoriented or injured. They have been known to remain in trees for days, which can weaken them and possibly result in falling from that height, particularly in strong winds. If your cat hasn't come down after a day or so, it may be time to take action.