Like a lot of people here I go by comments on this forum, particularly by those people whose tastes consistently match those of my own.
I also tend to do research on authors, specific books etc. but when I discover an author I like, especially when they have produced work that I like over a period of time in those cases I'll buy the book without thinking about it e.g something new form Tim Powers Christopher Priest, Gene Wolfe, Neil Gaiman etc..
I also read reviews on the 'net', stuff from TLS, NYR etc. book podcasts e.g. Bookworm in the US and Arts and Books Daily in Australia and wiki is useful for me as a starting point when researching specific works or 'literary' movements etc. plus reading academic articles. attending book festivals, talks and so on.
As my reading interests extend well and truly beyond SFF and Horror I've also found over the years to trust books associated with specific imprints/publishers like Harvill, Pushkin Press, NYRB, Penguin (black) classics, Norton, Library of America, Oxford University Press, Dedalus, Alma Classics, OneWorld classics etc. I also pay attention to specific 'awards' like the Nobel Prize as problematic as that can be/is as I've found generally speaking the authors still tend to be of a fairly high quality, certainly worth investigating and some SFF awards like The World Fantasy Award are pretty good indicators of books worth investigating.
Similarly in SF and Horror, as I have a tendency to collect entire series, I've followed the Masterwork series (SF and Fantasy), Wordsworth Horror series plus some specific perennial anthologies.
All in all I've found this approach has led me to reading and acquiring a library of which over 90% of 'stock' I'll probably never end up recycling.
I don't tend to be swayed by swank marketing promotions, what the latest 'hottest' read in bookshops or popular press appears to be, the appearance of a particular book cover etc.. my interests lie much more in the quality of the content.