A few of my patients have nightmares of their teeth falling out! It's actually quite distressing when your veneers crack and drop off.
First of all, make sure the veneers are well bonded onto the teeth. This is a very technical aspect of the treatment. It needs to be done well.
After the veneers are well bonded onto teeth, their strength to withstand chewing forces are actually quite high. The day to day function of your teeth should not post any problems but night grinding and clenching can break the veneers. Thus, if you grind your teeth at night, you should wear your night guard.
Porcelain veneers do not stain easily unless there are micro cracks or rough portions. Therefore, please do not use abrasive toothpaste (for people who smoke) as they contain many rough particles.
Another important point to note: when you go for scaling and polishing with dentists, ensure they use special polishing paste designed to polish porcelain veneers e.g. Proxyt. Normal polishing paste contain some mild abrasives. Please check with your dentist about what they use.