If you're applying for undergrad I don't know if work experience will really matter. It probably wouldn't hurt to put it in assuming it fits into the guidelines specified by each school but for the most...
The Graphic Design Forum is a social forum where designers can gather and talk shop or discuss many non design related topics. It's also a place where members can give and get advise on design related issues. It is at it's core though a forum full of designers of all skill levels and various design related professionals. With that understanding it really makes no sense to fill your signature with what amount to advertisements of design services. The saying "trying to sell ice to Eskimos" sums it up nicely.
To those of you who post links just in the hope that the search spiders will follow them don't bother. We have no follow attributes inserted into every link. All that will happen is the text will get indexed. But it won't get any extra crawling done on your website.
We allow members to post one link to their portfolio in their sigs if they so choose. That can come back to haunt them in the long run though. We do not allow logos, links to services, ads for services, links to various social networking sites that showcase services, blogs or any other forms of borderline advertising.
Use your signature space to quote your favorite book or song. Use it to express your creativity in whatever manner you wish.