Data entry form design for mobile/tablet

Simple is easy to say but isn't always easy to do. I started designing web UIs on a computer like 18 years ago and I still find it challenging to keep forms simple, even just forms for entering database records.

What's a well-designed data input form you've seen on mobile or tablet, and more importantly what exactly makes it a good design?

What are some principles good designers apply to keep it from being a just a big wall of form fields?