There are two techniques for growing sugar crystals. The most common one involves making a saturated sugar solution, hanging a rough string in the liquid, and waiting for evaporation to concentrate the solution to the point where crystals start to form on the string. The saturated solution could be made by adding sugar to hot water until it starts to accumulate in the bottom of the container and then using the liquid (not the sugar at the bottom) as your crystal growing solution. This method tends to produce crystals over the course of a week or two. It fails if you live someplace where the air is so humid that evaporation is very slow or if you place the container in a location where the temperature fluctuates (like a sunny window sill) so that the sugar stays in solution.
If you have had problems with the simple method, here's what you need to do.
Grow a seed crystal.
The other way to get a seed crystal is to break one off from a piece of rock candy or other sugar crystal. Use a simple knot to tie the seed crystal onto some nylon line (don't use rough thread if you have a seed crystal). When you suspend the crystal in the solution you want it to be completely covered, yet not touching the sides or bottom of the container.
Supersaturate your crystal solution.
You need as much sugar as possible to dissolve into solution. Increasing the temperature dramatically increases the amount of sugar that will dissolve, so you can get a lot more sugar into boiling water than in hot tap water, for example. Boil the water and stir in more sugar than will dissolve. It's a good idea to pour the solution through a coffee filter to make sure no undissolved sugar remains in the crystal growing solution. You can use this solution as-is or you can let it evaporate for a day or so, until you see crystals start to form on the container. If you choose to evaporate off some of the liquid, reheat it and filter it before introducing the seed crystal.
Cool the solution slowly.
Sugar becomes much less soluble as the temperature falls from boiling to room temperature or refrigerator temperature. You can use this characteristic to stimulate quick crystal growth. The 'trick' is to allow the solution to cool slowly because if a sugar solution cools very quickly it tends to become supersaturated. This means solutions that cool quickly will become highly concentrated rather than grow crystals. You can slow the cooling of your solution by setting the whole crystal growing container inside a pot of near-boiling water. Either seal the crystal growing container so that no water gets in or else make sure the sides of the crystal container are tall enough that water won't get inside. Let the whole setup slowly drop down to room temperature. Sugar crystals grow slowly so while you might see growth within a couple of hours, it could take a couple of days to be visible. Once the solution has slowly dropped to room temperature, you could continue to take it down to the temperature of the refrigerator (if the container will fit inside).