Hi Sunny,
Here is what I do;
First I vacum the whole KB {lighly with a soft vacum brush-becarful that you don`t scratch the display} Then to clean the key`s I {spray a small amount of "glass plus" or "windex" on a soft cotton cloth and wipe the key`s , gently [back to front try not to wipe left to right] . After the KB is clean I always keep it covered when not in use and to protect the display I cover it with a [well washed , soft polishing cloth. you must make sure that it is realy free of any waxes or silicons that might be on the cloth] The kind I use is just a plain cotton polishing cloth folded in half and placed on the display before I cover the KB.
There are a few places to by covers, right now I`m using the one that "Yamaha" has to offer it`s a little large but "ok".
The only down side to using "plastic or vinyl" covers it that if you store your KB with the cover on you could damage the plastic case of the KB. But if you chose somthing "less taylored" you could just use a [soft "beach towel"] or an old [cotten sheet] the idea is to keep as much dust and debri from falling in the "speakers" and interfearing with the "switches"
Take care and happy playing
jedi