Hi Simon,
The longer the lengths you can keep it in, the better. Then it is important to seal any end grain as soon as possible. If wax is used, it needs to be hot wax, so it is not the most convenient of jobs to do. However, there are easy, and just as effective, alternatives. I use undiluted PVA, brushing it on liberally with a stiff brush, making sure that I have fully covered any end grain (including and points where side branches might have been cut off). Equally, you could use any left over paint - either gloss or emulsion will do. Store it out of rain and sunlight to dry it - although you might want to turn some of it wet.
Depending on how you you think you will want to use the wood, to release some of the stresses that will be inherent in the wood (which in themselves can lead to checks (splits)) and to assist drying, you could cut the wood lenthways through the pith before storing.
Hope this helps.
Cheers,
Trev.