I have experimented with a few different local materials, but my all time favorite is Red Osier, and I use it almost exclusively. It really does make amazing arrows! As for my method of processing, I usually collect in winter, peel the bark off after a week or two, and hand straighten periodically while the shoots dry. Once they are dry enough to stay relatively straight, I bundle them up and put them away to be made into arrows the following winter. When I first started doing it this way, I had to wait a year for the shafts to 'cure', but since I collect a new bundle every winter, I always have a year-old batch waiting to be made into arrows.
I also dowel my osier shoots so that they are 11/32" parallel, rather than leaving them tapered. At 11/32" X 28" with a 125g point, my arrows always come out over 500g, and usually between 500-550g. I have never used a spine tester, and find that 'spining' by hand is sufficient. Once you figure out what your arrows should 'feel' like, it is easy to match others to that particular spine. And Osier tends to be a very forgiving shaft material, flying true from bows of varying poundage.