Every kilobyte should contribute something to the message.  Avoid flashy designs that require huge files to be transferred. 

It's always a winner to rely heavily on text and images with small files (re-sized or compressed if necessary). Keep navigation simple and intuitive. Eliminate the superfluous.
Why is this important? Because if your site is a bandwidth hog, it puts more pressure on every part of the system. Your readers will need more powerful computers and higher bandwith connections.The host will need more diskspace for storing the files and more capaacity to handle traffic.Complex sites mean that more things will go wrong. You will have to put more resources into fixing and maintainig the site.

One other idea: before starting your own site, see if you can meet your needs by contributing your work to an existing site.   Having your own site is a lot of work - why not cooperate with someone else, so there are fewer, higher quality sites?

Bart / Energy Bulletin http://www.energybulletin.net

We're not only carbon neutral, we're actively reducing our impact. Check out the energy and water saving, and waste reducing, efforts that our datacenter is doing. Then, we offset what we can't reduce. http://www.sustainablewebsites.com/green-data-center

As for energy consumption of web hosting, the big question is probably more companies like google and yahoo, who have huge server farms.  I remember reading that there was some consideration of putting huge server farms in places like North Dakota, where you could hook them up easily to wind farms.  Pushing for something like that would be the low-hanging fruit with servers.

ALSO, Smart website design is really important.  The author (and anyone else) should think carefully about who the website is intended for & what information it should convey.  Keep it simple, keep it easy to navigate (and include the meta-info like keywords).  That will limit the energy use of people visiting the site.  (And really, you could be HELPING reduce energy use by making it easier for web users to find the info, therefore spending less time searching the web for it.)

Try this test: plug your monitor into a Kill-A-Watt $20 measuring device. Tell me if you see more than 1 watt difference between a black screen and a white screen.

A modern hosting server with intel dual core processors consumes 60-70 watts per hour. The air conditioning is another 20-25%, switching equipment 10% and power conversion loss 8%. Total under  100 watts per hour. This server can easily handle 500 or more personal web sites.

So the average personal web site consumes .2 watts per hour .48 watts per day and about 15 watts per month. Quite a bit less than one "wall-wart" that is connected to something that is turned off.