before getting signature links just post comment on forums.after doing that they will allow signature link..It's very selfishness that without posting comment on forum to get signature link from it.
.com, .org etc are easy to get, you can just buy them. .edu's however are not easy to get: only schools, colleges, libraries (i believe) and other educational agencies are allowed to have a .edu link some rules applies to .gov, and therefore .gov backlinks are also more valuable than .com etc...
I hate to burst your bubble folks, but .edu and .gov TLDs are worth no more to a search engine than a .com .net .org .info .biz or other TLD. What makes them appear to be more valuable (and remember, the appearance is a mirage) is that these sites often contain valuable information that other sites are more likely to link to, thus inproving their standing in the search engine result pages. This isn't always the case, especially when you consider that nowadays most of the pages on .edu sites are user generated, meaning put up by the students, you know, like MySpace pages. But for those pages that DO have quality information that is valuable to the general public, you WILL see people linking to them, and it's the multitude of incoming links to those pages that will boost the appearance of value in the search engines, not the type of top level domain.