Humans have a need for closure and derive satisfaction through seeing things completed. Designers can use this to their advantage when designing their products and applications by communicating overall progress and whats next. An example is websites that show the percent completion of profile information.  While they may only require 40% of the information to be completed they would prefer it all to be completed.  They motivate the user to do so by presenting the percent completed towards the end goal. Celebrating completion of goals is another way to motivate users.  Games do this often by providing digital medals or trophies. 

Audiences view certain people as authority figures. While authority figures vary by industry and by topic, associating your product with authority figures adds credibility. This is the reason why tech projects seeking investment focus so much on promoting the team behind the project.   

Status Quo Bias

Humans rely on recognition and heuristic shortcuts to make daily decisions. They usually work well and save a great deal of time and mental energy. But as decisions become more complex and more difficult, it's harder for us to rely on a simple shortcuts and still maintain confidence in the outcome. Instead, when complexity arises the human brain relies on other things to help simplify the problem such as default values. Default values are usually perceived as being based on others past experiences, and when people don't have our own past experience to rely on they tend to rely on the experience of others. Decisions can be influenced by conveying what options are most popular or preselecting default values 

Challenges need to be of appropriate difficulty for the reward the user receives.  Challenges that are too difficult create stress and deter users from completing them.  But challenges of appropriate difficulty can engage users.  It's important to recognize that a user's skill level will increase with time, so challenge difficulty must be adjusted and balanced along with a user's learning curve.

Too many options overwhelm the a person's ability to arrive at a decision. Whenever a process or design can be simplified it should be.  Tunneling is one way to control the number of option presented to a user at one time and to influence their behavior.  A multi-step "wizard" is an example of tunneling. Tunneling closes off branches of a process to focus the user's attention on a simplify task at had.  It guides them along a specific path and removes the chance of detouring.