Wow, what a great thread!
Some thoughts: 1) Regarding Brent's question on whether a vendor deserves an answer from the decision-maker they've been interacting with, I have found (when operating as the client) that it has generally been well received by the vendor when I directly and specifically share what we liked about the product and sales process AND what made us either hesitate or ultimately need to say "No."
2) As a vendor, we try to value-add all along the process - if we end up not being the best source to deliver a project/service to a client- we refer to our competitors. It's painless and we've received call backs for other projects when we did this - because we focus on preserving the relationship. Whenever I've been in too much of a hurry to "get the proposal out" rather than thoughtfully add-value in some fashion - I've always ended up regretting it.
3) When doing a cold call or an initial presentation (after inquiry) we often send a "coffee break in a box" - sending a bottled Starbucks frappucino, a few individually wrapped biscotti and our materials. We share that we appreciate that the client's time is valuable and that although we would love to have a coffee with them in person, we invite them to enjoy a coffee break on us and review the materials when it suits them. We have typically received a warm reception to this; however, there have been times - when we didn't get a response at all.
4) I think vendors have to think of themselves not only as solutions-provider (which has been pointed out in the posts) but also as a partner (walk in the client's shoes). Don't call on Mondays, on the first of the month, or push on in your presentation - when you see the potential client sighing or when they are sending signals that show you need to G-O (go!).
Bottom-line - in this harried, hurried world - people appreciate when instead of looking out for #1 - you look for ways to be a resource or help without attaching a price tag.