Use a basting brush dipped in water to brush the sides of the pot where the sugar starts to build up. Keep disolving the sugar build up with the water, but be careful not to use too much water, it will delay cooking time. Another method is to cover the pot, slightly cracked, in the hopes that water will condense on the pot side and run back into the pan, disolving the sugar.
I prefer the brush method.
The reason we do this is to keep the syrup from having a crystaline seed, the the syrup doesn't cool to a crystaline mess.
Also, don't let the syrup, get to a full foamy boil, but bring it up just to the point of boiling.
Start out with a recipe of 2C Sugar to 1C water.
If you follow these guidelines, you should end up with a sugar syrup that will last almost forever without crystalizing, fermenting or molding, but will still have an ideal density for cooking and baking.