Neil, I'd suggest you find yourself a copy of a good text on woody plant propagation -- I'd recommend The Reference Manual of Woody Plant Propagation by Michael Dirr. You should be able to find it at your library or can order on Amazon.

There is a science to growing trees from cuttings -- you can't just take a twig and stuff it into soil and expect it to grow. Timing of the cuttings, size, heat, humidity, light, the need for a rooting hormone (or not) all come into play. And some plants just do not take well from cuttings at all and require other methods of propagation. In general, aspens and other members of the Populus genus are typically easy to root from cuttings but timing and methods are everything with regards to the success of this practice.

You could also pose your questions on the Propagation forum here on GW. There's a lot of very experienced growers there who might be able help further.