I have undertaken to create such lists as an academic exercise myself, and quite a lot of effort is involved in doing this. There are a variety of approaches that I will suggest, and I would place more emphasis on the first approaches.
Literature review papers: Look for literature reviews on the subject of interest. Read the reviews' analysis and discussion of the state of research, there should usually be a discussion of how the state got where it is, and their citations should point you right at the seminal works.
Paper Cites: review known papers of interest for their citations. You'll want to recurse back to this strategy as more papers become known to you.
Textbook tables of citations: Review several respected textbooks that cover the subject, and inspect their citations for more works that you want to be aware of.
Reputation, reference: Ask professors and PhD students in the field what papers they think are important. Some professors may not be particularly helpful, but some may drop a comprehensive bibliographic database right in your lap.
Seminar Reading Lists: Ask for the reading list for PhD student seminars in the field.
Most Downloads/Read: This might not lead you to seminal works, per se, but seminal works will tend to be more read, and this provides another way of categorizing and prioritizing your review the literature. For example, if one paper has been downloaded at a rate 1000 times higher than another on the same topic, you might choose to examine the former first.
Also, you'll need to keep track of your efforts. Expecting to solely rely on your memory is not only wrong, but likely to mean you've wasted much of your research time. I use Zotero, a free bibliographic database with integration in Firefox, to track the papers I've read. If I get papers or books in electronic form, Zotero can also store the electronic copy for me. It can also quickly create a table of references for you, and has some other nice features. It also has competitors that I'm not as familiar with, of note are Mendeley, a freemium model platform, and Endnote, a rather popular and mature commercial platform.