I think fans can do a heck of a lot. I've seen fans pull off some pretty sophisticated campaigns in an effort to save their shows. Also, every year the fans like the Star Wars fans and the Browncoats have booths at the sci-fi conventions to promote their groups and the movies/show they love.
You know what I would love to see a fan do? Dress up as a giant I-Man DVD for these sci-fi conventions. What a great advertisement that would be. And then when people come up and inquire about the costume make sure to have plenty of flyers on hand about the DVD and where folks can buy them.
There are, of course, easier things that fans can do. I know many I-Man fans are doing something as simple as buying copies of the DVD and giving them as gifts to everyone they know.
The very best marketing is just plain old fashioned word of mouth...people getting excited about a show or movie and talking about it to their friends and family. And with the advent of the web, fans now have an opportunity to reach out to a huge amount of people.
Universal isn't going to spend marketing dollars on old shows. Marketing is extremely expensive. Any revenue they gain from additional DVD sales would be negated by the cost of advertising. It's just not cost worthy for them. So they put the first season out on DVD and wait to see what kind of sales it gets. That affects their decision as to whether season two gets a release and when. Of course, shows like I-Man have a long tail, so sales revenue may be spread out over a longer period of time. This would just mean we have to wait longer to get the next season on DVD.