For storage of the unused part of the 500g package as long as it stays cold and dry you should be okay. A zip lock vacuum bag or a food saver bag would be ideal.
Harvesting the yeast after fermentation might be a good idea:
Or you could propagate the yeast and pitch part of it and store the rest. There is a good link for how to do that on this site. I'll try to dig it up, but someone else will probably beat me to it.