Balancing Your Range

Whenever you're in a hand, you should always be adding up information and trying to put your opponent on a range.
Many players, however, neglect to think about their own range or perceived range.
This is a mistake. Unless you're playing with complete droolers, your opponent is going to be trying to put you on a range as well.
It's up to you to play unpredictably and make it difficult for him to accurately pinpoint your holdings. You do this by mixing up your play and balancing your range.
The basic idea is that you never want to become too predictable. You want your opponent to know that in any situation you can have any hand.
If your opponent can never accurately put you on a hand, in the long term you will get the most of your battles.
Avoiding Predictable Lines
Say you're playing with an extremely straightforward player. If he flops top pair he will always call down; if he has anything less he folds.
He never raises his draws and he never raises unless he has top pair, top kicker beaten.
Against this sort of predictable opponent you can easily deduce his likely holdings. It won't take you long to know almost exactly what he has at any point in the hand.
That's because he takes very predictable lines. If he has top pair beaten he raises marked cards; if he doesn't, he just calls. Even the most unobservant opponents are going to figure that style out eventually.
It should be obvious why you don't want to play in such a straightforward manner. Your opponents will always just fold when they are behind and call when they are ahead.
When you are so easily read, your opponents can play perfect, mistake-free poker against you.
"Balancing" Your Range
Basically balancing your range means this: whatever action you take can be interpreted by an astute opponent in many different ways.
A few years back in online poker lense, play was a lot different. Players would only reraise you if they held AA, KK, QQ and maybe A-K - but that was a stretch.
This style of play became very easily read. The good players would pick up on it and were able to fold even JJ to a reraise.
As the game progressed, players started three-betting lighter (with worse hands) to exploit other players' folding tendencies. What they actually started to do was three-bet with a more balanced range.
No longer did those three-bets mean they only held a big pocket pair - instead they meant they could have anything from AA to 5-6s.
Since this range was "balanced," no longer could their opponents fold all but their best hands to the reraise. Now they were forced to call reraises with a wider range. This led to the light three-bettors getting more action on all of their three-bets.

Expanding Your Calling Range
Let's say in your online six-max game you play a tight-aggressive style.
You're more prone to flat-calling raises in position than you are to three-betting them. If a player from the cut-off raises, you're likely to call with K-Q, A-J, 99, good suited connectors, etc.
If you always call with that range, your play becomes fairly predictable. Your opponent will realize that if you just flat-call his late position open, you can never have a monster.
To combat this, occasionally you will have to call in position with a monster - for example, AA, KK, QQ. This balances your range.
Now your opponent has no idea what you cold-called with ... it could be 7♥ 8♥; it could be A♠ A♣.
Adding to Your Check-Raising Range
Another example is check-raising. A lot of amateurs make the mistake of only check-raising with a monster hand.
This is a mistake because an astute player will catch on and fold everything but his or her best hands - the exact opposite of the goal of the check-raise, which is to get value.
A better way to check-raise is by doing it with a more balanced range. Stop check-raising just your monsters and start to add more hands to your check-raise range - hands like flush or straight draws, top pair hands, etc.
When you check-raise a more balanced range it helps in two ways. At first you are going to take down a lot of pots with semi-bluffs.
As your opponent catches on, he'll start to realize that you don't only check-raise the nuts and he'll start to look you up. Then he will start paying your check-raises off because he isn't sure if you have the nuts or complete air.
By check-raising a wider range your play becomes much more difficult to read. No longer can your opponent put you on an exact range; instead he's going to be left guessing as to what you really hold.
This is your entire goal in balancing your range. You want to make it difficult for your opponents to deduce an accurate range. When they can't put you on an exact range, they'll be unable to make perfect decisions against you.
If they can't make perfect decisions, they're going to be left making mistakes more often than not. And mistakes are like money in your account: every time your opponent makes one, you profit.
In short: avoid playing any one type of hand a single way; instead make sure that every play you make could have a variety of meanings.