The best way to sharpen your knowledge is to work on real problems and, if you want to build a proven portfolio of your projects, machine-learning competitions are a viable place to start:

• Kaggle: This is the number one competition platform, hosting a wide variety of challenges with large prizes, strong data science community, and lots of helpful resources. You can check it out at https://www.kaggle.com/.
• CrowdANALYTIX: This is a crowdsourced data analytics service that is focused on the life sciences and financial services industries at https://www.crowdanalytix.com.
• DrivenData: This hosts data science competitions for social good at http://www.drivendata.org/.