My cleaning product of choice is "rain"...
I don't put anything on the panels, nor do I advise my customers to do so. Most are a few stories up on a roof; a place where I really don't want to see people crawling around, not to mention that usually the majority of panels is unreachable unless you crawl over them (another thing I tell people not to do).
Unless you live in a place that sees very little rain, or very infrequent rain, I honestly don't believe there's much to gain by manual cleaning or putting stuff on them. There was a study done in central Spain, to see what the effect of dust settling on panels was (central Spain is pretty arid, with a dry season and lots of dust). They could see the effect of dirty panels, wasn't huge though, and 10 minutes of rain took care of cleaning them back to pristine state again.
We have another pollutant that is much more persistent and seriously degrades solar PV output here, it's called "snow". Just a light dusting will kill all PV output, taking the array from 6kW to 300 Watt. Many do clean it off panels here, some have contraptions like a modified pool-cleaner with a squeegee at the end, to reach two floors up. In general I tell people to make peace with the loss of production because of winter snow. For grid-tie there's not much to be made or lost during those times anyway, though for off-grid that may be different.
-RoB-