You must be flexible and be able to make changes to your game plan as the game progresses. In other words, don't get in a mindset that what works on one player will work on all players. This will not be the case. Unlike the computer, live players learn to adapt quickly. A certain tactic you use against a player usually doesn't work in the same game over and over.
Human opponents learn quickly.
They will be more prepared the next time you try the same tactic. Your well thought strategy may work the first time around, but usually fails if you try using them more than once against the same player. It's best to have several different strategies and become an unpredictable player. Keep your opponent guessing all the time. It's very frustrating to new players who have never played online to find they've lost their base in the first 10 or 15 minutes of game play, or worse still, the first two minutes, which does happen frequently due to engineer rushes, etc.