Never copy and paste code.

There's no magical reasoning for this. It's quite simple actually. When you innoncently think "oh! I'll just copy and paste this code here, make a few changes and I'll be finished!", it rarely works out that you actually REMEMBER to make all of the changes that were necessary.

Of course, I'm not talking about the copying and pasting associated with pieces of code that should be put into a module, but with pieces of code (perhaps even one if statement that you want to make into four) that "almost" do what you want, but not quite.

The one situation where I'll use copying and pasting is when writing stored procedures in SQL. It's much faster to copy and paste table names and columns (one at a time usually) than it is to keep retyping them.