You can never learn enough when you're practising design. The Summer of Design 12-week email course from designer David Kadavy, based on his book Design for Hackers, is currently free if you sign up in the next 12 days. The course, which we've featured in the past before it was removed for a while, takes a reverse-engineering approach to design, helping you understand the reasons behind good design. The course is divided into 12 lessons, each one delivered by email to your inbox once a week.
Once the free enrolment period is up, the course will cost $US9 ($12), while the book it's based off of costs $37.73. Still, it's hard to beat free.