I can't say it enough.  Think at Trick One.  Make a Plan.  Do not play too quickly. 

When there is a trump suit, count the tricks you will lose, and decide if you will draw trumps right away.

At notrump, count the tricks you will win, and decide what cards you might need to knock out and if you have enough stoppers.

 More than half of all failed contracts are lost at the first trick.

 Deciding in which hand to win the first trick, or whether to duck or win it, is often key. 

Larry's Webinars on Declarer Play