The second best way is to review their books.
Goodreads is one of the easiest places to leave a review. You don't even have to write anything, you can just rate the book.
On Amazon, you have to leave a short review along with your "star" rating, but it only has to be 20 words (which is two less than the last sentence). You can leave something like:
★★★★★ I really loved this book. I couldn't put it down and didn't want it to end. Can't wait for the next.
★★★★A great book. Not once while I was reading it did I feel as though I was going to hurl.
The point is, a few moments of your time after you've read something you liked can be a huge boon to the author, particularly one who has chosen to self-publish. They put lots of time into their work so you would have something to enjoy. Please take a few moments to give a little something extra back.
Think of it as an incredibly inexpensive holiday gift for an author. We are so easy to buy for....