One of the talks that stood out to me from Flutter Festival London was this one by Marcos Sevilla, where he covers the importance of layering with regard to your Flutter app's architecture.
Sometimes I think it's easy to get lost in the details of a Flutter project, and I really like what Marcos has done here to sort of back up and look at some of the bigger-picture issues regarding architecture. I think these would be especially helpful if you're just getting a new project off the ground and want to start on a solid foundation.
I've mentioned this resource before, but for new readers I thought it worth reminding that Google Developer Expert Mangirdas Kazlauskas has put together this cool little Flutter app to highlight some design patterns which Flutter developers might find useful.
These patterns run quite a gamut, including those in the creational, structural, and behavioral design pattern categories.
I think these might especially come in handy if you're struggling with making an architectural decision about some aspect of your application. These patterns can help bring clarity to the tradeoffs you might be making with the approaches you're considering.