The first thing my wife noticed when I unpacked her new 20" iMac was the glossy screen; she was very unhappy. She had gotten used to the matte finish of her 17" iMac and did not like looking into the "mirrorMac". Knowing that Apple did not give us any choices, I looked around the web and found a product that got good reviews from Photodon that was just over $40; so I ordered it. It came quickly and i tried to install it this weekend. Since it looked a lot like the screen protector I have for my iPhone, I ignored the instructions and proceeded based on the good and bad experiences I had with the iPhone's screen.
DISCLAIMER - these are my experiences and your mileage may vary. If you have never installed one of these silicon protectors before, or, if my experiences don't sound like your own previous experiences, then you should follow Photodon's instructions (the screen is not returnable once you start to lift off the backing, so make sure that you understand 100% of his instructions before you even think of starting).
All told, after unpacking everything, it took less than 15 minutes. First, I blew any random dust off the screen with my breath and then waited about 5 minutes for any dust kicked up in the area to settle. Then, being careful not to kick up any remaining dust, I cleaned off the screen with the yellow microfiber cloth supplied; I folded it into fourths and dragged it lightly across the screen horizontally back and forth, and then top to bottom; the mirror finish helped me to see any dust, dirt and smudges still on the screen. I then held the protector (with backing intact) to the iMac screen to make sure I knew how it would fit. Then I cleaned the screen again.
Then, bring very careful, I peeled off an inch or two of the bottom of the protector's backing and centered it against the bottom of the iMac screen (there is a slight flare around the black screen that you can hold the protector against). I pulled the protector on and off the bottom several times until there were no bubbles (holding it by the still-backed top of the protector). Then I grabbed the end of the backing paper and slowly peeled it off in one smooth motion, letting the protector roll off and stick to the screen as I went along (if it doesn't look like it's rolling off, stop now and retry). I had to pause a few times when large bubbles appeared and pulled the protector up and then let it roll back onto the screen. When I was done, there were a few medium bubbles and a few small ones on the screen.
I then went after each of them. I could tell that one was a [damned] dust particle that made it's way under, so I peeled the protector up from the nearest corner, carefully pulled the dust particle off and gently "rolled" the protector back onto the screen, being careful not to introduce any new bubbles (stopping and retrying when I did). Each of the larger (air) bubbles were handled similarly -- I peeled the protector up from the closest corner, pushed the bubble out (from the non-sticky side of the protector) and then gently rolled the protector down into the screen -- as Photodon recommends, a credit card works wonders here.
Within a few minutes, the job was done and the results are phenomenal. I would highly recommend getting one of these protectors from Photodon (they don't have an affiliate network -- rest assured that I get nothing from this endorsement). If you haven't worked with silicon screen protectors before, I would recommend either getting some experience putting one on your iPhone first or asking a friend (who has done them before) to help. Although you can certainly do it yourself without experience, you are likely to become frustrated (as I was the first time I put one on my iPhone); you get much better every time you do it (this is my third experience -- two iPhones and one iMac).