Code is clean if it can be understood easily – by everyone on the team. Clean code can be read and enhanced by a developer other than its original author. With understandability comes readability, changeability, extensibility and maintainability.
General rules
Follow standard conventions. Keep it simple stupid. Simpler is always better. Reduce complexity as much as possible. Boy scout rule. Leave the campground cleaner than you found it. Always find root cause. Always look for the root cause of a problem. Design rules
Keep configurable data at high levels. Prefer polymorphism to if/else or switch/case. Separate multi-threading code. Prevent over-configurability. Use dependency injection. Follow Law of Demeter. A class should know only its direct dependencies. Understandability tips
Be consistent. If you do something a certain way, do all similar things in the same way. Use explanatory variables. Encapsulate boundary conditions. Boundary conditions are hard to keep track of. Put the processing for them in one place. Prefer dedicated value objects to primitive type. Avoid logical dependency. Don't write methods which works correctly depending on something else in the same class. Avoid negative conditionals. Names rules
Choose descriptive and unambiguous names. Make meaningful distinction. Replace magic numbers with named constants. Avoid encodings. Don't append prefixes or type information. Functions rules
Don't use flag arguments. Split method into several independent methods that can be called from the client without the flag. Comments rules
Always try to explain yourself in code. Don't use closing brace comments. Don't comment out code. Just remove. Use as explanation of intent. Use as clarification of code. Use as warning of consequences. Source code structure
Separate concepts vertically. Related code should appear vertically dense. Declare variables close to their usage. Dependent functions should be close. Similar functions should be close. Place functions in the downward direction. Don't use horizontal alignment. Use white space to associate related things and disassociate weakly related. Objects and data structures
Avoid hybrids structures (half object and half data). Small number of instance variables. Base class should know nothing about their derivatives. Better to have many functions than to pass some code into a function to select a behavior. Prefer non-static methods to static methods. Tests
Code smells
Rigidity. The software is difficult to change. A small change causes a cascade of subsequent changes. Fragility. The software breaks in many places due to a single change. Immobility. You cannot reuse parts of the code in other projects because of involved risks and high effort. Opacity. The code is hard to understand. 
Principer