How to Clean a Porcelain Veneer?

When you clean a veneer, use a soft bristled tooth brush, and gently massage the front side of the veneered tooth. Dental plaque doesn't adhere to porcelain the same way it does to enamel, so it doesn't require the same amount of time and strokes of a toothbrush that enamel requires. 

If at the dental office, they should use a special paste made for porcelain, which is usually a very fine diamond polishing paste on the porcelain if they use anything at all.

You pose an excellent question. Whereas properly placed and polished porcelain veneers should be relatively maintenance free, there are some things that dental offices and you can do to extend the live of these veneers. Using a non -abrasive toothpaste is better for the veneer and your teeth as well.  For preventive appointments, it is best to polish the veneers with a paste designed for porcelain such as Proxyt.  

I think the jury is still out there on whether a cavitron or other instruments can damage the veneers at the margin of a well done veneer.  I have seen some of my cases doing great after 18 years.   All in all, be safe and ask the hygienist to use products designed for porcelain veneers.