I am always evaluating new software for my company and find that vendors are more willing to provide a copy if you willing to take the time to provide a written feedback on their product. I make it a point of talking to them half through my evaluation. I am also up front with them in explaining that I am looking at other similar products. If the vendor start bad mouthing their competition, then I am pretty sure I don't want to do business with them.
At the end of the evaluation period, I will usually send each of the vendors a summary of my evaluation. I am please to say that many of vendors appreciate the feedback I have sent them even if their products where not selected. I have seen some my comment incorporate in their updated version.