Since anti-statists are the underdog, there is an understandable bias toward discussing all the factors that grow the state in the ever-present creep of fascism. This may be effective for propaganda purposes and for keeping libertarians vigilant, but it is not epistemically ideal to discuss the one side without the other. It also opens up libertarians to weaknesses and blind spots in debate.
What are some things that keep state power from growing faster than it does? Since war is the health of the state, why doesn't the state continually launch false-flag attacks to keep people in a perpetual state of panic (even more than now)? What exactly happens when the state overreaches? If cops just get paid vacations whenever they abuse their power, why don't they do it even more - like by planting drugs on their enemies?
Why anarchy fails
1. The will of the population
2. The will of the politicians
3. The awareness of both parties as to what the government is doing
Ultimately these are the only things which can restrict the growth of government
Going along with Neodoxy's response, I think "public trust" is the essential thing. The governments of the Soviet Union and its satellite countries ran rampant over the people there. But they ultimately collapsed. Why? From what I've read, the people totally lost the motivation to cooperate with the governments except (perhaps) when directly threatened by them. The governments themselves finally reached a point where they weren't willing to oppress the people any further. Here I'm referring in particular to the events behind the fall of the Berlin Wall, where no one in the East German government wanted to order the soldiers guarding it to shoot the people who wanted to cross over it. Apparently that was because none of them wanted to take responsibility for a massacre.
The keyboard is mightier than the gun.
Non parit potestas ipsius auctoritatem.
Freedom of markets is positively correlated with the degree of evolution in any society...
This is where I'm more of an optimist. The statist order is fighting gravity to keep the system going. The whole thing is a scam, an inter-generational pyramid scheme, a cultish delusion. The whole house of cards can collapse with the snap of a finger. In fact, when you look at the history of governments objectively and do not treat coups as part of a continuous history of a single "nation", you will find that most governments on Earth are less than 70 years old and many die every year.
Most people are anti-statist in at least one way. I love chatting with people and finding their anti-state peeve. Some people hate cops. Some people hate war. Some people hate the partisan political charade. Some people hate government-protected animal experimentation. Some people hate the government-protected banking racket. And so on. There is a variegated patchwork of piecemeal expressions of anti-statism.
The key - as Rothbard identified and which Rockwell has been hammering on for decades - is to bring these people together and show them what they all have in common: the State! And, further, to show them that there are solutions to our problems that are not the Mad Max doomsday scenarios that government propagandists constantly try to scare people with (remember the "government shutdown" stunt during the Clinton era?) The 10th amendment Marijuana, guns, etc. acts show the way forward on this. Nullification. Agorism. Privately mediated agreements (judge.me). Alternative digital currencies (GoldMoney, Bitcoin, etc.) Cryptographically protected privacy (PGP, TrueCrypt, SilentCircle, etc.)
But these strategies are not a "right-wing" strategy and we need to be careful not to let the State apologists paint them out that way. It is crucial for anti-statists who originate in the right-wing to reach across the aisle to people who originated in the left-wing and have become disillusioned with politics-as-usual (Gitmo, anyone?) We need to bring them on board and build a durable, full-spectrum coalition whose particular aims may vary widely while sharing in common a basic stance of ending the perverse effects of government intrusion into ordinary life.
"No army can stop an idea whose time has come." Our weapons are more powerful than guns, bombs and tanks. Our weapons are honesty, history, reason, education, cooperation, peaceful non-compliance with dictates, secession, nullification, privacy, mutual agreements, industriousness, frugality, stewardship and, perhaps most importantly, patience.
Don't get lasso'ed into the temptation to effect quick change. No change worth having will happen quickly (until its time comes, then it will happen faster than you can say "boo"). Rather, while you are waiting for "things to change", occupy yourself with your own affairs. Keep busy and diligently arrange your life to achieve your personal goals.
It is frustrating that the warmongers are creating this environment where it is so dangerous to freely express our basic humanity in simple ways like trading with one another or starting a family but it will pass. WWII begin in 1938. It was over by 1945. Seven years of total war is a lot of total war but - in the grand scheme of things - seven years is not that much time. They can only keep the current level of intensity up for so long. The war on terrorism, the financial crises, the climate change garbage, the shakeups of the Middle East and the fracturing of the EU. Something's gotta give and it will give sooner rather than later.
The expansion of the State is checked by opposition from without (i.e. from the masses), and opposition from within (competition between various factions or institutions within "the State"), thus the weakest State is non-democratic (less legitimacy in eyes of masses) and decentralized (high level of intra-State competition).
What are some things that keep state power from growing faster than it does?
Besides the other good answers here, an obvious one is society itself. That is, the general trend of opinions in society. The trick is that the trend doesn't move at a consistent rate in any direction—tip the scales too far and a critical mass of people will see the matrix.
The answer to your other questions regarding why X doesn't do Y as much as they can is that X does do Y as much as they can. Any more and the scales would start to tip toward a critical mass.