See what work has already been done on your topic. Bibliographies are listings of citations of previous work on a topic.
Browse the listing of dissertations to see what kind of work is being done at the doctoral level.
Papers are the main method of disseminating knowledge in linguistics. Exploring previously written papers can help you see what is expected and how people present their work.
Books are often a great source of inspiration or data for your research topic.
Book Reviews give you a detailed look at someone's personal, professional opinion on a book. These are helpful in many ways, including: showing common objections to methodologies, understanding what makes a strong case, and identifying flaws in argumentation. Often the reviewer will refer to other published works or provide a bibliography of his own, which can be yet another source of information.
The LINGUIST List's own project, MultiTree, graphically displays language relationship hypotheses and allows you to visually compare them.
Another project by The LINGUIST List, LL-MAP, allows users to search digitized maps by region, country, subject language, author, or publisher. Users can also upload their own maps from data they have collected.