
This is one of those areas that beginner level poker players have a lot of trouble with. They don’t raise preflop enough. You will even find some extreme cases at the micros where someone is only raising as little as 2% of their hands (basically AA, KK, QQ and AK)!
This kind of thinking is completely understandable though. Why risk more than the minimum especially with really speculative hands like,

Why not just see a flop as cheaply as possible and if you don’t hit, fold with a minimal loss? All valid points.
But the problem with this line of thinking is that poker isn’t a game where you only make money when you have a good hand. The beautiful thing about this game is that people very rarely have anything good. So there is an enormous amount of “dead money” just sitting around waiting to be had most of the time.
Good, winning players know this and seize the initiative before the flop. If they end up making a hand, awesome. If not, they have given themselves another way to win the pot as well though; by taking it down preflop or by following up with another bet on the flop.
Secondly, when the pot gets limped preflop people are far less likely to play a big pot with you. They are more inclined to play their hand straight forward and fold when they miss. They do this because they are thinking the exact same thing that you were thinking: “I’ve invested such a small amount, I’ll just fold and move on.”
However, when you raise preflop that gets some money in the middle on the flop. People are far more likely to want to fight for this. And ultimately they will make big mistakes fighting for it.
However instead of rattling off a bunch more arguments in support of raising preflop why don’t we just look at some cold hard facts from HEM?
Go into the “reports” tab and go,

Add in these values
- VPIP = True
- PFR = True
This will ensure that HEM only shows hands where you voluntarily entered the pot and raised. You can filter by a specific stake if you wish. I used a 700k hand sample from NL2 from this year, 2011. My filters look like this.

Now find your winrate when filtered for these scenarios. Here are my results.

As you can see I am winning at 113 BB/100 when I voluntarily play a hand and raise preflop with it. Let’s have a look at the reverse now. I will change my filters slightly so that PFR = False instead of True. It will look like this.

Now let’s have a look at my results again over this same sample when I limp or call a raise instead.

As you can see I am only winning at 58 BB/100 now. My winrate when limping or calling preflop is about half of what it is when I raise! Now to be fair my hand strength is generally going to be a lot stronger when raising as opposed to limping or calling. However I don’t think that this is enough of a reason to explain the massive difference in winrate.
The real reason goes back to what I said before about there being a bunch of dead money out there waiting to be had. Here are my winnings when I raise preflop but do not see a showdown.

And here they are when I limp or call.

A big reason for the difference in the winrates above is my inability to win the pot when I don’t have the initiative. I am actually losing money in these situations when it doesn’t go to showdown. However when I raise preflop I am picking up a lot of pots even when I have nothing because I have initiative and people give me credit.
This is all the “proof” that I need. When in doubt, raise it up.