Experiment with whatever flowers you have available. I hope you have started adding herbs to your garden. Avoid using plant material that has been sprayed. You don't want any ingredients in your potpourri that could be harmful. Since the fragrant oils in flowers and leaves fade, fixatives may be added.
Orris root is a commonly used fixative.
Others are oak moss, clary sage leaves, coriander seeds, bay leaves, cloves, mint leaves, or dried citrus rinds. Generally you will need to add one tablespoon of fixative per pint of flowers. You may use more than one fixative in your recipe. Spices and herbs are added to further enhance the scent. Some that work well are cinnamon, rosemary, thyme, lemon balm, and allspice.