Because I may have many tins open at a time, I have developed this method for retaining the freshness of a tin over months.
It works with both round and the cuboid tins that dont screw down and are more likely to dry out.
As I open the tin, I carefully take out the inner package, then I use Glad Wrap, Cling or one of those other cellophane plasticky things that is used for wrapping food.
I lay this in the tin with an excess of about 3 inches around the perimeter.
I then put back in the baccy, with its original wrapper on top of this.
This excess wrap I then fold, before closing tin.
Its a bit of bother, but I find that it will keep the stuff nice for months. Also the baccy is not in direct contact with the plastic.
I also use old tins with the original wrapper, to store my mixtures using this method. Better than any pouch.
I'll post pics of this on my blog
http://pipesmagazine.com/lonleypipes/