Espalier is the name given to a technique of growing trees into a two dimensional flat plane by grafting, pruning and training. The espalier technique was developed in Europe in the 16th century to help temperate climate fruit grow in cooler climates, by growing them flat against a sunny wall. Gardening Australia has a great Fact Sheet on Espalier.
Any shrub or bush can be espaliered, but popular species include: Olives, Citrus (orange, lime, lemon), Camellias, and Gardenia.
Finding the location and setting up is the most challenging part. The tree will need to be grown on some kind of sturdy trellis. If you choose a wall, simply attach some horizontal tensioned wires about 30cm apart, they are going to have to take some weight so ensure they are well secured.
Some kind of clips will be required to hold the branches being trained to the wires; soft spongy plastic ties can be bought at your garden center, or you can cut up any old nylons that have a run.
Before you plant your tree, look at it from all angles. It is best to plant it with the most horizontal branches to line up with the wall. The other unwanted branches can be pruned back to keep the tree flat.
You prune the tree down to the first cordon (that is at the lowest point), cutting just above two buds that are on either side, forcing new branches to grow. Trim all but the two desired cordons and one vertical branch that will go up to the next cordon. When it is well above the next level, trim again just above two buds, forcing multiple branches once again. Continue this pattern until you've reached the top cordon, at which point you will only leave the two side branches. This method will take about 4 years for your branches to fill out to three cordons, but you'll have branches pretty much right where you want them.
There are a bunch of different styles of espalier you can try....some of the most common are:
For more tips and ideas check out the Lucky Duck Farm, they espalier many of their fruit trees.
And to see a step by step process check out some of these videos!