Readers and writers have a symbiotic relationship; one can't exist without the other. The absolute best way to support the writers you love is to buy their books. But that's not feasible for everyone, all of the time. There are plenty of great ways to support the writers you love without breaking the bank.
Use your library. Yes, authors benefit from libraries, too. Libraries know what titles by which authors are circulating widely, and will often buy more copies if the hold list is deep. If your local library doesn't have an author you love, ask if you can make a purchase suggestion. Most libraries are happy to make acquisitions based on patron needs, and even if they don't have funds to buy a new copy, they may be part of a consortium that will find a copy for you, and deliver it to your local branch. This helps writers by showing a demand for their books. And I imagine we're all pretty familiar with the theory of supply and demand.
Put Our Books Face Out on the Shelves. Any shelves - library or bookstore. How many times have you been browsing a shelf and your eyes have skipped right over a spine? A lot, I'm guessing. If you see a book you love, put the cover art face out so that it can shine. Think of it as a book recommendation to a stranger.
Follow Us. Love or hate social media, it can be the coin of the realm. A big following doesn't equal success, but it is a small indication to the writer that they are doing something right, and somebody cares enough about their book to see what else they have to say. Sure, it's a bit of an ego stroke, but if we're having a bad writing day and log on to Twitter to see ten new followers... no lie, it helps.
Tell Us Our Impact. I occasionally get fan email that opens something like this: "I don't know if you even read your own emails, but..." Yes, I read my own emails. I'm not nearly a big enough deal to not read my own email. I also answer them, which sometimes surprises readers. I appreciate each one of those emails. The bad writing day I mentioned before? A few of those have been assuaged by a message telling me what one of my books has meant to the reader.
Tell Someone Else About Us. Word of mouth is still the most effective marketing there is, and money can't buy it. If you love a book, tell your friends. Tell your friend's cousin. Tell anyone you know who loves to read. Those little ripples can turn into waves that sustain an author's career. It doesn't mean that we're all going to be New York Times bestsellers, but it does mean that our backlist might stay in print a little while longer because a new reader just discovered an author that flew under their radar up until now.
Write reviews. Did you love one of my books, but not the next one? (If so, you're not alone in that). Either way, I want you to write a review. Reviews - good or bad - on shopping sites like B&N or Amazon show the site that people are reading the book... which means they are buying and interacting with it and it is therefore worthy of promotion. Amazon needs to see a minimum of 50 reviews on a book before it will begin suggesting it to readers in the "Customers Also Bought..." section.
Note: There is some debate among writers as to whether or not this is true, but Amazon is notoriously tight-lipped on their alogrithms. Regardless, think of your reaction when you see a book with 13 reviews versus one with 256... you automatically know that the latter is doing better.