Firstly make sure any seeds are thoroughly dry. If they are loose keep them in a sealed envelope so you can label them & write any instructions on them. An open packet can be kept in a sealed envelope too. then put them away in an airtight container. just make sure they are not kept in an area where there may be humidity, i.e your kitchen, laundry.
Seeds can be kept in a zip-lock bags too but be sure the to keep them in a cool dry place, if the bags sweat it may cause mildew on your seeds. They should be fine to use next year.
You can preserve them in bag vacuum sealers or air-tight canning jars for an year or more but keep these seeds dry first. Before preservation, put these unused seeds before sunlight for few hours, though, you can even use an alternative method of drying so choose as you wish.
I'll recommend bag vacuum sealer as the best preservation medium. Besides, try to choose any non-humid location where you are going to store these seeds.
Thank you, hope this answer could be helpful for you.