F-- error for Nikon 105 F/2.8 non D

I just picked up a used Nikon 105mm Micro. (non D version) and when I connected it to my D90 I received the F -- error.  I did a quick wipe on the contacts with my finger and reconnected the lens and after a few reconnects, it worked fine.  I used it today then all of sudden got it again.  I again wiped off the contacts with my finger and it worked again.

What is the proper recommended cleaning method do fix this permanently going forward?

Thanks for the help.

Reid