I have used willow shoots or limbs anytime the sap is up and growth is visable. I have always taken the small shoots cut them into pieces two to three inches long and then put into a quart jar. Then use rain water or pond water filling it to the top set it aside and let it work for a week or so. After it has worked I then use it for watering other plants, this is a great way to start roses. I have done it this way for 25 years. Hope this helps

Thanks for the advice. So I basically need non-tap water to do this? I guess bottled spring water will work - I don't want to use creek or pond water because it might turn into something more like a biology experiment if there are microorganisms in the water...anyway I will go out and get some shoots soon and see how it works.

Any suggestions for preserving the liquid, or will it lose its effectiveness after a while?

The auxin in willow is simply indoleacetic acid which is degraded rather quickly by both soil and water microorganisms. For most folks it is more sensible to purchase a rooting hormone preparation which contains indolbutanoic acid. It is equally effective and is not greatly affected by microorganisms. I have had pleasing results with one of the Hormodins(1,2,or 3). Peek at the internet!!!