How to preserve herbal syrup or herbs that will be used to make syrup?
Remember that whatever method of preservation you use you will always lose some flavor, there's no way to keep it all. 

If you want to make a syrup for long term preservation then freezing them is your best bet unless you want to add preservatives which you don't really need to do if you have a freezer. A smart way to do it would be to pour the syrups into ice cube trays, then once frozen pop them out into zip lock freezer bags so you can use the trays for other things, and keep the ice from sublimating. I'm in favor of straining the syrup first, that way the cubes are ready as soon as they come out of the freezer. 

Depending on the herb it may be more efficient to simply freeze the herbs directly. Thick herbs like sage, rosemary and thyme will do just fine while others like basil and parsley will turn to mush and you'll lose flavor. 

Have you considered drying them? It's not hard to do, and there's plenty of how-tos out there, may be the best solution for thin herbs.