Honestly, it’s a really deep topic — for a while I majored in interface/interaction design in school— and getting good at it is like getting good at writing. It’s not like amateurs can’t write solid stories, but they probably don’t really understand the decisions they’re making and the factors involved, and success usually involves accidentally being influenced by the right combination of things at the right time.
The toughest hurdle to overcome as a developer is not thinking about the gui as a thin client for the application, because to the user, the gui is the application. Developers intuitively keep state in their head and know what to look for in a complex field of information, and often get frustrated when not everything is visible all at once. Regular users are quite different— think about what problems people use your software to solve, think about the process they’d use to solve them, and break it down into a few primary phases or steps, and then consider everything they’d want to know or be able to do in each of those steps. Then, figure out how you’re going to give focus to those things… this could be as drastic as each step having its own screen, or as subtle as putting the cursor in a different field.
Visually grouping things, by itself, is a whole thing. Important things to consider that are conceptually simple but difficult to really master are informational hierarchy and how to convey that through visual hierarchy, gestalt, implied lines, type hierarchy, thematic grouping (all buttons that initiate a certain type of action, for example, might have rounded corners.)
You want to communicate the state of whatever process, what’s required to move forward and how the user can make that happen, and avoid unintentionally communicating things that are unhelpful. For example, putting a bunch of buttons on the same vertical axis might look nice, but it could imply a relationship that doesn’t exist. That sort of thing.
A book that helps get you into the designing mindset even if it isn’t directly related to interface design is Don Norman’s The Design of Everyday Things. People criticize it like it’s an academic tome — don’t take it so seriously. It shows a way of critically thinking about things from the users perspective, and that’s the most important part of design.