Poker is not simply a game of cards or a game of luck. A big component of the game is bluffing. At the end of the day, you may have the worse hand on the table but with a little bluff, you can still end up the big winner. Perhaps, this is where the term poker face comes from - the ability to keep a straight face. This is particularly helpful in keeping the other players unknowing on what lies on your hand.
However, once your opponents have mastered your poker face, it is time to mix it up. To become successful in poker, you have to be unpredictable. If you have mastered the art of giving mixed signals and keeping your partners confused, you may want to do the same on the poker table. Keep the players guessing.
In online gaming, players will not have the added benefit of seeing each other's facial expressions. Bluffing can extend to written communication - the online chats during the game for example.
Bluffing does not only refer to your physical expression (or in the case of online gaming, written communication). It also refers to the way you play your hand. Because of the myriad of variations, there is really no strict formula or textbook cases to follow. It would be suffice to remember that if you keep playing your hand the same way, your opponents can easily read and predict your next move giving them the advantage.
To cite an example, if you have in the past three consecutive games attempting to do a check-raise bluff, your opponents will likely remember this move (assuming they were paying attention). This is no different from the boy who cried wolf. If you attempt to do this move again, they will be suspicious. But if you must insist on doing so again, you better make sure you have a strong winning hand as they would likely call on your bluff.
At the same time, you would also need to keep an eye on your opponents and try to read their moves - the same way they try to size you up. And if they are good players, you should look out for your opponents waiting for the opportune moment to give you a taste of your own mixed signal medicine. This is what makes poker double the fun. You need not only recall what your opponents are up to, you also have to remember your own play.