Besides making sure that you are static free when you handle a MOSFET for other static sensitive device, you need to make sure any other surface that the part makes contact with, like your workbench top, also does not have a charge. I use a antistatic mat on my workbench but I've worked in the Semiconductor industry for a long time and have just gotten used to using them. Also keep other static generators away...like a roll of scotch tape and other plastics that can hold a charge. Lacking a mat you can use an antistatic bag to keep the static sensitive parts on.
I was in a IC testing area once watching a tech test some parts. She tested a part wearing grounding straps, but then accidently dropped the part on the floor. She picked it up and retested it, and it then failed for input leakage, the classic sign of electrostatic damage which was later confirmed by failure analysis. The input leakage was not high enough to cause it not to work however. But I put this part into a short burn-in, and the leakage increased until the part no longer functioned. This is why they refer to static damaged parts as "the walking wounded". Some people think that they never have zapped a part but later that part can become unreliable and it never gets associated with a static zap.
and once gone, its gone (until you move and create more static, again).
