I'm constantly thinking about ways to instigate use. I think that ultimately comes from my laziness, so I'm constantly trying to trick myself into action. I suppose if I can trick myself into doing something then I can use that same incentive to get a user to do something on a site.
Why the need to motivate? We all hate to be told to do something. We are all driven by novelty and discovery. We are all driven by prestige. We all get bored and need something to motivate us. We get bored by tedium and change helps us stay engaged. We are all human.
Everytime I install a new app, it tries to direct me through all of the features by walking me through a short tutorial. I know it is trying to be helpful, but I honestly just skip through it. I don't think I've read one of those usage tutorials ever. Sure they are helpful, but I just can't seem to muster the patience or need to go through it, so I rely on my own sense of discovery and the intuitiveness of the app/site to teach me how to interact with the site. Most sites have controls that are based on existing controls of other popular sites - so in most cases the extra effort to go through the tutorial is mute.
A better approach is to design the app/site to walk you through the process with a small task - like helping you through your first post or setting up a profile. I remember reading about how the first Super Mario Brothers Level was meant to let the user discover all of the controls without seeming like a tutorial - letting the user learn through discovery. That type of learning is called Constructivism.
It is interesting how living things thrive on novelty. Cats will chase a laser point. Babies will play with a box for hours. Many times the novelty is spurred on by the activity of discovery. What's that sound? What's that smell? Why is that moving? What is under there? Usually novelty wears off, so of all of the ways to motivate a user, it is usually the one that has to be changed the most.
As a designer, we use graphics and wording to add novelty. Usually we have beautifully designed UI elements like buttons and banners that are designed to entice users. Button elements are designed to look like they can actually be pushed. Usually we use colors that are high contrast so they can easily stand out. We place them in areas that are at eye level - so they are easy to see, or place them grouped together to make it easy to click them in succession.
Another way we create novelty is with animation and movement. Perhaps we can make a button blink if it needs to be clicked. We can also use a roll over animation to let the user know it can be clicked. If we want a user to see something it might change - which might be the case for a slider or a banner. All of these elements provide novelty.
Things to avoid when designing novel elements: Once a user is aware of an element, provide a way to remove it, if it slows down interaction or could become annoying. For example if you have music playing - give them the ability to shut it off. If something is blinking - give them the ability to stop it.
Also be aware that novel decisions can drastically be effected by the device that is being used. If a user is on a mobile device - toggle video usage - as not to use up all of a user's data plan. If a user is on a mobile device then roll over actions may not activate since the cursor isn't on the screen until actually getting pressed. There are many ways a user can reach your site - just be aware of what kinds of obstacles they might encounter.
Of all motivators, prestige, class and rank are the best. Users want to be better than their peers and in many cases they will do whatever it takes to get that recognition. Prestige is a recognition that comes from visible trophies to signify experience and excellence. In the real world this might be shown with badges and medals - like in the military with medals to signify rank or with boy scouts and their patches to show certain types of skill acquisition. Many popular video games use badges to convey prestige. The entire Xbox platform has levels and achievements built into the user's account - visible to anyone who views the details of the account. Prestige / class / rank can easily be built into any website that includes user profiles or social activity. You can create loyalty points that show how often a user logs in, purchases or reviews products. If an app or a site has any social functions you can start to include those badges that show how distinction. This is a motivator that encourages interaction and encourages positive behaviors. Websites like Reddit use points (karma) to encourage good posts and good conversation. The same system also curbs bad behavior by allowing the community to take points away when a user does something negative. Many forums websites will give some kind of profile distinction when a user has posted X amount of times. Perhaps the will have a badge next to their profile handle or maybe a title like "guru" next to their name which gives community klout to their posts. Another example would be leader boards which show how a user fairs compared to their peers. Ranking users against each other can motivate users to act.
Sometimes getting someone to interact is as easy as telling them to do something. We do that automatically with commands and wording that tells a user to do something - "Click here" "Submit Query" etc. But how do we tell a user to do something if they aren't in front of the button in question? We can solve that problem with a notification of some sort. In the case of a website we might use an email. Emailing the user can engage them into going back to the site - or clue them onto some kind of interaction. Some possible emails we might want to send to a user is:
Email is a perfect solution for a website or app, but applications for mobile devices have Notifications that they can leverage as well.
Notifications or push notifications look like a text message and show up on your device in some kind of notification area. These are instantaneous and can get users attention much faster than an email. The only draw back to using this type of notification is that it requires additional permission and possibly be limited by the device.
Text messages are another way to motivate users as well - It is the same as the push notifications except you are communicating through their mobile text messaging. It is has the benefit of getting attention quickly - but it has the draw back of being limited to just a text based message, that needs to be short, and can incur messaging charges.
Humans are creatures of change. We get bored with same old things and one of the surest motivators is to demonstrate change. People are drawn to things that are different and new to them. Changing the way things look helps to keep engagement high. Spring Cleaning keeps things fresh and new.
In regards to a website - 2 years is about the standard before requiring an updated look. This not only keeps things changing, but it also allows us to present the site with current design trends.