Working primarily as a software developer what resources have you found to improve your UI design skills?

Having always worked primarily as a software developer I have a good understanding of CSS/HTML but a poor understanding of what constitutes good UI design if I'm approaching something from a blank slate. I feel I've got a good understanding of good UI design when I see it and can combine pre-existing elements well but starting from scratch doesn't come naturally to me.

For someone aiming to understand the fundamentals of UI design and design in general,

• What topics would you recommend learning about?
• What resources would you recommend learning from?
• Which leaders in the field would you recommend following?