First you must cut the tree down to the point where only the main trunk is left (and maybe a branch, if it is solid), at a height where it is still at least a foot in diameter. Please don't ask me for techniques for tree trimming, I was making it up as I went along, and probably did some horribly dangerous things. If the thought of being 50ft up a tree with a chain saw scares you (I was shaking), let the professionals do it. If there is space for a bucket truck to reach the tree then the pros can do the job better.
The bark needs to be removed so the holds can be placed solidly. My Silver Maple was easy, a year after the main trimming the bark started to dry up and loosened. I used a spade to remove the bark that was still sticking. Use a safety top rope when you are on the ladder doing this operation, for instructions skip ahead to the top anchor construction section. After the bark was off I found the wood surface had many small nubs and spikes that would have ripped skin in a fall that slid down the trunk. I used an electric hand sander to smooth the trunk, a rasp would probably do as well.
The top anchors are the most important part of the entire system, don't skimp on them. They need to be massively strong, easily inspected, and redundant. There may be other ways to do it, but I trust my system.
First, buy enough 3/8 in. chain to circle the trunk plus a few links, circumference + 10 in. should do it. At my hardware the chain is rated for 2600 lbs., looping the trunk gives an almost 5200 lbs. rating.
At the point where the chain will circle the trunk, determine which side of the trunk will be the best place for the rope to hang. Screw in a couple of 4 in. long 3/8 in. Lag eye bolts 180 degrees apart, at 90 degrees from the intended rope placement. The chain will rest on top of these eyes, not go through them. Wrap the chain around the tree and pull it tight. Join the two links that almost touch with a 3/8 twist link, leaving an equal number of links hanging past the joining point. The rope will go through the two end links of the chain. Use screw links to secure the chain to each eye. Place two screw links through the chain at the opposite side from the joining point to act as TR pulleys for the other side of the tree.
A few words about hardware store lag eye bolts. There is much temptation to simply screw a couple of them into the trunk and run the rope through them. This is not safe. The point where they will have the most stress is just under the wood, out of sight. Only use them to keep the chain in place, not to hold stress. In my system as the chain takes weight it cinches tight around the tree, placing only a fraction of the force on the eyes.
I coated my chains with a clear coat of polyurethane to prevent rusting. You might also try an oil based treatment, but whatever you use make sure you can still see the metal surface for cracks.
The belayer must be anchored for a safe belay. I use the post of a chain link fence on one side, and the leg of my kid's play tower on the other side. Both were made with steel posts set in buried concrete.
A similar system could be made by digging a hole, placing a large steel pipe so it was flush with the ground, then filling with concrete. Drill the top of the pipe for a cross bolt, then sling the bolt. If this is done right it will allow you to remove the sling and run the lawn mower across the top of it.
A more movable system uses 5 gallon buckets. Some sort of steel loop is placed in the bucket, then it is filled with concrete, leaving the top of the steel for daisy chaining. I have no experience with this construction but I think there are instructions out there.
The bucket system was used at the old AYH wall where I learned climbing, and although it worked, I didn't really like it. The buckets would lift with a heavy climber or long fall, and would sometimes end up on top of the belayer's toes. Any anchor system that uses movable objects - concrete blocks, logs, etc. - will have the same problem.
BARK BOXES AND WILDLIFE
The base of the tree will have the grass worn away after a few climbing sessions, Rain will leave a muddy start well after the tree is dry enough to climb. I boxed the tree with 6 in. sq. pressure treated lumber, then removed the grass and dirt inside down four inches. I used the bark I had scraped off the tree to fill the box.
Fungus of various sorts have tried to grow on the side of my tree. I just scrape them off, I don't think they could be a problem unless allowed to grow.
The tree will immediately become host for all sorts of insects. None of them has bothered me enough to reach for the bug spray, but the number of bore holes has increased as the tree has aged. I don't think the bugs would be an issue for the structural safety of the tree until it is many years old, inspection of old dead branches taken off the tree showed the bugs only bored through the top 1/2 inch or so. I have been thinking about spraying the tree with rotenone in order to prevent the bugs from getting out of hand.
Termites are a different issue. I have seen no evidence of them, but they are a threat. Allowing them to become established would threaten all other buildings in the area. A professional exterminator could drill around the base of your tree and inject chemicals to stop termites. My personal dislike of having these chemicals in my backyard has made me take a different tactic. If I see any evidence of termites, I will cut the tree down and dig out the base.
Woodpeckers have twice tried to make homes in my tree in the early spring. They bored holes in the softest, oldest wood, at the very top of the trunk above the top anchor chains. Both times they never nested, my guess is that the urban environment was too disturbing ( I saw sparrows in the holes after they left, maybe they drove the woodpeckers off). The size of the trunk where they made the holes is so large that I don't think they are any structural threat, and anyway they are above the chains. You might just move the chains down and cut off the top part of your tree if they try to nest below the chains.
Don't climb on a tree that you don't trust. I suppose there are ways that you could rig cables up to winches and do a real stress test on the tree. As my tree gets older, I might try this. For now, I feel comfortable because of the size and solid feel to the tree. When I am at the top of the trunk it has no flexing or cracking sounds, no matter how hard I try to shake the tree. My guess is that if your tree makes any kind of cracking sound when you test the anchors, it is time to lower the chain. If the sounds come from the base, it is time to cut down the tree.
There is some flex on the top of the branch I use for the overhanging route, but no sounds. I will probably lower the chains on the branch this year to the point where it has little flex. I have been thinking about a system to back up the branch with a cable that goes from the tip back to the main trunk.
Inspect every link in the chains for cracks at least twice a year. Unscrew the eyes an inch and look for cracks in the shank and the eye. Thunk them with the heel of your hand to see if they are loose.
Anchor the belayer solidly. You don't want to meet him halfway up the tree after a hard fall.
Use a dynamic rope. Read the archives of rec.climbing if you want to get both sides of the issue, my feeling is that using a static rope will cause trouble sooner or later
If you have children in the neighborhood, they will try to climb the tree without ropes, no matter how much you tell them to stay off. I have removed all holds from the tree below the four foot level, this is very effective in keeping the young kids off. Older kids can figure out how to drag something up to the base to stand on to get started. I tell these kids that I will help them learn climbing if they want, but they have to follow my rules - no climbing without a rope. The threat to ban them from all climbing if they are caught breaking the prime rule has worked so far.
If you are going to have kids climb on the tree, or any non-family member, you should have a release of libility form signed by the climber or their parents. I am working with a climbing lawyer friend on this form, I think there are examples out there on the net. You have to be balanced in this document, don't scare people off with harsh warnings, but cover your legal ass. Gym climbing has a low rate of accident, and this kind of climbing is close enough that the rates of injury should be similar. My guess is that the rate of injury from play on standard playground equipment will be higher than the rate from roped tree climbing, there is always a kid or two every year who breaks an arm at the local school.