1. MY OWN WORK - In the indie market, I've done a sequel (PLAY IT AGAIN) as well as a short story series (FOREVER FINLEY). It's essential to mine your own work when you decide to do any kind of follow-up to a previous release--and not just to remind yourself of the order of events or minor characters' names. It's essential to get back into the feel of previous book--to remind yourself of the rhythm of the sentences, the overall mood. Those seemingly "minor" details will make your readers feel as though they're picking up where the story left off--which is exactly what you want a sequel to do (it's far tougher than you'd think it'd be).
2. OTHER AUTHORS' WORK - This is all about mining for new technique. I love, love, love authors that make me think about my work in a new way--maybe a book's not told in chronological order, or it's got a great pace or cool plot twists. The ways other writers put their work together constantly give me new ideas, push me to be better.
3. GOOGLE / YOUTUBE - I'm not talking about the usual fact-checking we all do for our books. I'm talking how-to. I've become the queen of how-to Googling in the past couple of years, especially as I went indie. I've Googled how to format an e-book, how to compile using Scrivener, how to do page numbers InDesign, how to create a cover for a print book...the list goes on and on. I've also begun to illustrate my own work--but my previous experience with any artwork had been with pen and paper, paints and canvases. Digital art was brand-new to me. So I've been watching a ton of illustrators' and artists' vids on YouTube.
There are so, so many talented people out there. Who are not content to hide the brush strokes--who want to show the process of making art, not just the final product. I'm grateful for their willingness to do so, every single time I begin to type a new "how to..." phrase in Google's search box.