Q: keeping iMac happy and healthy

Some tips here:

http://support.apple.com/kb/HT1147?viewlocale=en_US&locale=en_US

There are also software applications for this, most of which you can do yourself. The one that seems to be the most recommended here is onyX

https://www.macupdate.com/app/mac/11582/onyx

Do stay away from MacKeeper, if you search this site using that keyword ad you will find MANY posts regarding this.

There is not really much you need to do as the OSX takes pretty good care of itself.

Hope this helps

Mac maintenance steps

Most commonly used backup methods

See:

Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.