Get a screen protector. There are protectors out there made specifically for that make and model. I have one for my 798 model, I've had it for two years and haven't had to change it yet. I just use a homemade vinegar/water solution to clean it.
I use the blue transparent bottle "Monster Screen Clean" on all of my electronics - Boat electronics screens, LCD and Plasma TV screens, computer screens, etc... It's made for it. Works great.
All the best,
Glenn
From the Humminbird web site FAQ pages:
What products can be used to clean my display?
Humminbird recommends using Windex® Vinegar Multi Surface(in the spray bottle).Do NOT use any wipes products (which can also be available as the brand name Windex Multi-Task).
The lens contains a special coating which improves visibility. It is recommend to use the above cleaner because we have tested and proved this cleaner. It does not deteriorate the coatings on the lens. http://www.humminbird.com/FAQ.aspx?ContentId=1833