Syrups are made by boiling three pounds of Sucanat (dessicated sugar cane juice that can be bought at most organic food stores) in one pint of water until a syrupy consistency is obtained and then steeping the herbs in the hot mixture for twenty minutes to transfer the essential oils from the plant matter into the syrup. The herbs can also be simmered directly in honey or maple syrup for about ten minutes. Use two teaspoons of herb for every cup of liquid. Strain the syrup and store it, well sealed, in the refrigerator.
Similar to an infusion, or tea, a decoction is necessary when you are making remedies from tough plant materials, such as roots, bark, seeds or stems. To begin, place thinly chopped plant material into a saucepan and add cold water. Use 1-2 teaspoons of fresh or dried herbs to one cup of water. Bring the decoction to a very low boil, simmer for 15 minutes, and strain after the liquid has been reduced by one half. The lower the temperature of the boil the more intact the essential oil in the herb will be.
I generally don't bother with Syrups, but if for some reason you need or want to make one (some herbal syrups are delicious) remember that you can always add a tincture to a syrup mixture relatively quickly. Tinctures are highly concentrated and keep for a long time so they are my preferred method of herbal storage/concentration.
I generally don't bother with decoctions either. If I need to tincture an especially tough herb (usually a root or a bark) like rose hips I'll use a food processor or coffee grinder and grind the herb to a cornmeal texture and then use the tincture process on the meal. This is generally easier than a decoction and you don't run the risk of getting the mixture too hot and thereby spoiling the medicinal compounds.