1: Two micro fibre cloths (ensure that they havent been used for cleaning anything else, so they wont have any other cleaning solutions on them).
2: Moisten one of the micro fibre cloths with tap water and wring out as much water as you can (it should be damp not wet).
3: Wipe the screen.
4: Use the dry one to wipe the screen dry.
5: I also have a large camera lens cleaning cloth that I use to wipe off finger marks. I tend to give the screen a wipe with the micro fibre cloths once a month, and in between I wipe it over with the lens cleaning cloth or if I need to wipe finger marks off etc,
I also use the same cleaning method on my iPads screen too.
The cleaning methods I have described will give you a clean and shiny screen. And will do as good a job (if not better) than the expensive cleaning kits, which in my opinion are a waste of money.
Whatever you do, don't use glass cleaning solutions or cleaning solutions that contain alcohol. These can damage the coating that is on the iMac's screen!
Even Apple recommend cleaning the iMac's screen with soft cloths and water, see the below link.
First, IPS is not a coating on the glass. It is a technology imbedded into the LCD display. It stands for In-Plane Switching and it reduces the display's response time and clarifies the image when looking at it from off-center angles.
Second, the oleophobic coating is not so easy to damage that occasional use of alcohol to disinfect the display is a problem. The bigger prohibition agaist the use of rubbing alcohol is that it leaves a haze that is incredibly difficult to get rid of.
Just use a damp, lint free cloth and a little elbow grease. iKlear and other such products help but they can get a little pricey over the long run.
The newer 2012 and later iMacs have an oleophobic/anti-glare coaing just like the iOS devices. That would be the only coating to worry about. The older iMacs have no coating on the glass so it would not be an issue.
For smears and contaminants other than dust . . . . . dampen a soft 100% cotton towel/cloth with distilled water, wipe, then use a dry 100% cotton towel/cloth to buff to avoid spotting and water marks.
For dust, photographic lens quality micro-fibre cloth.