I recently read a great article by Cameron Chapman titled Mobile App Design Best Practices and Mistakes.
While reading it I realized I design a ton of mobile forms. So for that reason, I'd like to share some of my thoughts on mobile form best practices:
1. Add a few pages, scrolling a form is hard
Many people get nervous about adding a lot of pages in a form so they put a ton of fields on every screen. Obviously, your form should feel as short as possible. However, every time you touch the screen to scroll you could have pressed a button and moved to a new page. So don't worry about adding a few more pages. People will thank you while they easily glide through your form.
2. Don't forget about the keyboard
It's a common mistake to design your forms and forget about the space a keyboard or selector will take up. Always design your screens with those elements so you know how much space you have available. Plus, it makes it easier because you don't have to fill all of that room!
3. Default fields when possible
If you can set your field to a default do it. Just pick the most popular option and you'll save the majority of your users a click. People may accidentally select things but in my testing, that tends to be a low enough % that I'm personally ok with the error to conversion ratio.
4. Avoid optional info
I try to avoid optional info on mobile and desktop products but on mobile it's even more relevant because of limited space. There are certain scenarios where it makes sense to have optional info but in general, always try to think about what you really need so you can reduce friction. Also, since the optional info isn't required you can't rely on it the same way since not everyone will have it.
5. Carry data through
Since mobile forms are broken up into multiple screens you don't always have the luxury of being able to see what you submitted on a previous page. For that reason try to carry data through that was submitted on previous steps or in the past so the user can reference it while completing the rest of the form.