Great question to discuss. I have a slightly different take, hope this is helpful.
Sharpness is not something in a vacuum; sharpness is about your mind and intelligence as it relates to people in the outside world. Thinking you are sharp doesn't amount to much. Being sharp in situations is what counts, and will give you the thing you might be missing. So think of sharpness in terms of people. The only way to sharpen is to expose yourself to new things and test yourself against other people. Warren Buffet doesn't stay sharp because he plays chess, its who he plays chess with that's much more important.
1. work and socialize with smart and talented people, particularly ones that are considered top people in your field, find a fantastic team and work with them, look for people whose careers rely on their sharp minds, people that are a bit older than you, and slightly different.
2. become a better listener, try meditating to help with this. being able to clear your mind of extraneous thoughts will make you more open to developing the thinking you are looking for, it will bring you focus skills
3. think about developing skills which can result in your making more valuable contributions to situations. be curious about new things, hobbies, intellectual pursuits, also read incessantly, as over time you will accumulate much knowledge. tell stories well and be funny. you'll find yourself invited to join companies and social groups that provide what you seek, as people will always be curious about what you have to say.
