Gerry Bavaro of Didit says that you should first set a goal, then think about how to attain that goal (i.e. your strategy), and then pick tactics that can bring you closer to the goal.
For example, if your goal was to increase conversions by 50% in Q3, then the overall strategy would be to increase your ad's conversion rate. As Bavaro said, "If you increase your ad's conversion rate, you get higher in the results, you get more clicks and they convert at a better rate."
The tactics behind this would be split testing your landing pages, testing the ad copy and creative, and geography or time of day testing.