I expect that most of you know these. In which case this is just a refresher. There are, however, always one or two who will benifit from these posts.
Get your pieces out into the centre quickly. The opening is a race to see who can get their pieces out first while keeping at least a share of control of the centre.
This is the main point to remember; all the other rules are just footnotes to this one
More detail on winning the race:
Move pieces not pawns, and
move them to their best squares in one move if you can, and also
try to gain time if you can by aggressive moves.
Move minor pieces out first, not your Q or Rooks which can be attacked and lose time
Get a firm foothold in the centre and don't give it up
Generally move Knights straightaway to f3/c3 or f6/c6
Move your king to safety at the side by castling King's-side (which also gets your Rook into play)
Complete your development before moving a piece twice or starting an attack
Keep your queen safe. Don't move her out too soon.
Don't grab pawns or attack if you haven't completed development
What to do if there is a lead in development:
If you are ahead in development, start something going and open up lines for your better pieces
If you are behind in development, don't start anything and keep things closed until you have caught up
Do not sacrifice without a clear and adequate reason, eg.:
it secures a tangible advantage in development
it deflects the opponent's queen
it prevents the opponent from castling
it enables a strong attack to be developed
Wherever possible, make a good developing move which threatens something or adds to the pressure on the centre.
Why should you move the knights first? Well, knights are very much more effective if they are in the centre. (Bishops are more effective here too, but they can work from a distance). For the opening that has to mean Knights moving to c3 and f3 (or c6 and g6). Where should the Bishops go? The White King's Bishop on f1 could go to b5,c4,d3 or even e2. Which is best? That depends on what your opponent is up to. So, move your knights straight away to the centre, and while you are doing that your opponent's moves may suggest to you where you should put your bishops.