Each year deer cause millions of dollars in damage to home gardens. Their population has exploded in the past century as we continued the transition in land use from farm to woodlands and suburban communities. We displaced their predators, limited or prohibited hunting and introduced many varieties of plants which have served as a veritable banquet.
Generally speaking, deer eat about 2% to 4% of their body weight per day. For bucks, consumption is greatest in the spring and averages about 4 1/2 to 6 1/2 pounds of forage. In winter, the daily amount is roughly half of that. For does, peak consumption is in early fall, before breeding season. 
Deer tend to travel in groups. I have had as many as 10 - 12 in my suburban garden at one time! The goal of the home gardener is to alter their habits and encourage them to roam elsewhere. For me it has been a constant struggle and I have no 'permanent solution'. Like planning a vacation at Disney, deterring deer requires a battle plan, multiple options and flexibility. One has to recognize when one approach has failed and have another ready to implement. Last, there has to be an acceptance that there will be casualties.
The only truly worry-free, long-term solution to deer is fencing. The fence must be at least eight feet high. This can be expensive to install around the entire perimeter of your property and many local codes prohibit fencing taller than six feet. Where eight foot fencing is permitted, a more inexpensive option is to use heavy-duty, yet lightweight polypropylene (plastic) fencing. Dave Jensen, owner of Deer-Resistant Landscape Nursery in Michigan (Watchdog), says 'the black color of the fencing makes it less obtrusive in the landscape and tends to blend into the background, especially on sunny days.' He adds that the deer 'have difficulty judging the height of the fence and are more likely to simply move along, rather than attempt a jump'.
For suburban gardeners, this solution, even where permitted, may not be very appealing for complete perimeter coverage. This can be due to close proximity of neighbors and the need to install gates for the driveway. An alternative is to enclose selected areas inside your property to protect vegetable gardens or ornamentals that the deer are attracted to. Most homeowners should be able to do this themselves.
Another approach is to use various repellents. Most commercial repellents are sprays that come ready to use or as concentrates. Most are some combination of egg products, garlic, hot pepper and some agent that prevents washing off. They work by presenting a foul odor to the deer which will then, hopefully, move on. They must be reapplied regularly - sometimes even more often than the directions recommend, I have found. This can range from every couple of weeks to couple of months. Many people forget to spray the new foliage that is always emerging. They will spray their plant according to their schedule and forget about it until it's due again. I am usually spraying something every week. Most recently, I have been using Liquid Fence® (website), and it has worked reasonably well. You should make sure any product is safe for use on edibles if you intend to do so.
I have also had some success with Milorganite® (website), an organic fertilizer made from processed sludge from the city of Milwaukee. I use it in two different ways. I fertilize my lawn with it, which helps distribute it throughout my property. I also pour it into piles, about six or so inches high in spots along the perimeter of my property, concentrating on those areas where I have seen deer entering. My experience with commercial repellents in general is that they must be rotated because their effectiveness wears off as the deer get used to them. Just keep in mind that if they have their sights set on something, like one of my favorite lilies this past summer, they may still eat it, even if it was recently sprayed, as my lily was!
Other commercial repellents include predator (fox or coyote) urine, combination attractant / shock devices and water spray 'scare' systems. These are not effective because the deer simply get used to them. There are also commercial deterrent systems that are installed and serviced (renewed) by a contracted company. They tend to be quite expensive.
There is no shortage of homemade recipes and concoctions reputed to repel deer. This includes making a liquid similar to the commercial products using eggs, pepper, etc. They tend to wash off much sooner and must be reapplied every time the plant gets wet. Another is to use human hair in small mesh bags or stockings. This is not very effective and is downright silly in my opinion. Others include stringing fishing line, hanging mirrors or cd's to spook them, and even using human urine! Don't waste your time (or compromise your dignity) with any of these. The only one that I have tried that showed some success was the use of strongly scented soaps. You cut them in pieces and put them in mesh bags or stockings. Deer tend to avoid strong scents, but these only cover a small area and who wants these hanging all over the place? I have used garlic oil clips in a similar way, but again, it requires many just to cover one decent sized shrub and they look ridiculous.
There is one final 'homemade' deterrent that does not work very well but is worth mentioning. That is the one I like to call the 'deranged dash'. That is where the gardener runs full speed, arms flailing, expletives erupting, eyes bulging, toward the 'invading' deer, who obliges by practically yawning with fright. Survivors stranded on an island are less animated when they spot a rescue crew than are gardeners doing the dash. The deer will usually wait until you are about four feet away before scurrying away, leaving you breathless (so you can't even continue the profanity barrage) and looking for rocks to throw at the 'coward'. Not that I have ever done this, of course.
A third approach is the use of deer resistant plants. Deer, like people, have their favorite foods and the simple idea is to use plants that they don't care for and hope they look for something tastier. There are many desirable plants that deer will tend to avoid. According to Jensen, 'deer prefer a bland diet and will bypass aromatic plants and those containing irritants or toxic chemicals'. He leaves all his plants accessible to the many deer which roam his property. This affords him the opportunity to see firsthand if the plants are remaining resistant. Deer also shy away from course or hairy foliage, and some prickly plants like barberry.
Among the plants in my garden that have never been touched by deer are Alchemilla, crape myrtle, Amsonia, Cephalanthus, Euphorbia, ferns, hardy geranium, hellebores, blue spruce, castor bean and many more. Even the use of resistant plants has its limitations though. According to Amy Albam, master gardener and Cornell Cooperative Extension agent, 'even when we use more and more supposedly resistant plants, we are introducing plants that the deer may ignore until they realize that the material is palatable'.
There is one last approach that you will not read about anywhere. It is one that I have employed for the past few years. I buy my garden-envious neighbors plants that are deer candy, such as hostas, hydrangeas, tulips and lilies. When they come to me upset that the deer have ravaged my 'thoughtful gifts', I cheer them up with replacements. I consider this technique the equivalent of the fastpass in Disney. In love, war and deer deterrence, a little guile goes a long way!
So if deer are bringing you to tears and making you regret the day you started gardening, don't give up! Use a combination of the techniques described and you should be able to enjoy your Eden.
(Please 'mouse over' photos for captions, and look for upcoming articles on deer resistant plants!)
Sources:  Extension.org - Deer
Many thanks to Dave Jensen, of Deer-Resistant Landscape Nursery, and Amy Albam, Cornell Cooperative Extension agent, for their contribution.