Hi all. I've been on itch.io for only a few months and already have made some sales and have all my three paid listed items in the top 10% of 'popular' section. Here's my advice, broken down, for indie devs:
1) Make a creative, high quality product and price it very reasonably. I particularly like going for 'under served' niches and pushing pricing as low as I can justify in hopes this will boost volume of sales and ideally ratings/reviews, improve customer satisfaction, word of mouth from happy people, etc. There are a lot of indie devs here on Itch.IO, almost as many devs as players, and way fewer items in the Game Assets section than in 'Games' so I focused on asset packs out of the gate - which you can see here, with intent to launch my games ie. Miniature Multiverse, a bit later.
2) Promote like crazy online. Forums you're active on, social media, wherever you have a presence and following, use it to hype your project, and if you followed step 1, there won't be a lot of complaints about your mentioning it. I'm doing that now. And if you're thinking, 'selling makes me uncomfortable' then maybe you need to look back at step 1 again, make your product better for customers, until you actually know your product is good and a real bargain for customers, then hopefully you will be able to feel comfortable selling.
3) Visuals sell. If you have some cool imagery, put it front and center, to grab attention, and know animation can be very effective too if you want to go the GIF route.
4) Yes, let people donate. I've had under 20 actual sales [so far], but had multiple buyers buy my items and leave a tip. That happens more frequently when the item is free or dirt cheap AND also good, you'll find people will appreciate the work you did - hundreds upon hundreds of hours of it - and they'll tip more than you think they will.
5) Upsell with a discount - bundles are awesome, people will often click on one item, see it is part of a bundle, and end up choosing the bundle instead even if it costs more. Of course, they're also getting 2 or more products, not one, but it doesn't really matter to you as the 'cost per item' is negligible in a digital store!
6) Use the item itself as advertising. Including a note within the downloadable, like a PDF readme or something, works great. Add your website and itch profile links there, and include discount codes because nobody is a more promising prospect for future sales than people who have already bought one item from you and were pleasantly surprised by it.
7) Your itch.IO product pages should, when useful, remind people that you have other items on Itch.IO, or stuff in development. And better, you can post free stuff, little free samples or demos, that then grab traffic and direct people to the full paid version as an option. Many people will gripe at this, saying it is a paywall, but I think a small paywall is better for the players and for the game quality, than the inherently broken model of freemium nickel and diming. But if you want the full game playable without a payment, try this: Put ads on the loading screens in the free version, ads for your other products or other peoples' ads, but not the sort that aggressively and annoyingly interrupt play. (This is why a loading screen banner between each level, makes sense) and then offer a paid premium version with ads removed and some extras like the soundtrack, an artbook/making of video, etc. That way it feels more worth it. Incidentally, I'm trying this with 'Panoramic Worlds' about one year from now. There's an interesting aspect to that launch also insofar as the game is not finite, it will continue being expanded and the additions will happen faster the more often people are buying the premium version.
8) Make your game available to as wide an audience as possible, on as many platforms as possible. I love HTML5 for this reason, but it is also limiting in many ways and that's why I'm focusing on desktop systems Mac/Windows - for some graphics intensive games. But if you can make your game run in HTML5 more power to you!