Vetos answer is pretty good. I used to play soccer and so did my cousin. In high school I good hit 50-60 yard field goals pretty consistently and my cousin set the high school record for his high school at 55 yards and got offered a scholarship at Princeton.
One suggestion my cousin always used to make was to kick the ball the same every single time. The more consistent you are, the better.
For example, if he was kicking a 25 yard field goal he didn't kick it lighter. He's still kick it as hard as he could as if he was kicking a 50-60 yarder. He said it doesn't matter if it flies 20-30 foot over the goal posts. It just has to go between them.
By not kicking it lighter or harder and kicking it the same every single time he reduced the chance of shanking it to the left or right.
The best way to control your kick is with a soccer style kick. Use the side of your foot, not the toe. You get more directional control with the side of your foot. And on punts, it gives the ball more wobble, making it harder for the other team to catch.
It's probably best to turn the threads away from you. Having the flat side of the ball come in contact with your foot gives less opportunity of the ball going left or right.
Then you also want to watch your approach and the angle you come around to kick the ball. If you watch videos of guys approaching the football you can almost tell before the guy kicks it if he is going to shank the ball left or right. You want your body square on the ball in the direction you are wanting to kick it. Some guys will come around at too much of an angle. Or if they are kicking with their right foot will plant their left foot too far back then end up swinging their right foot too far around. Or plant their left foot too far forward then don't swing their right foot far enough.
When I kick, I like to kick so that when I finish up and my foot hits the ball my body is lined up looking right at the center of the goal post. I tend to do a curved approach.
Practice with a soccer ball. That works pretty good. We had a garage with a brick wall and an old corn crib that I could kick the ball against and didn't have to worry about denting or damaging the wall. I'd pick a spot out on the wall then aim for it. It's a good practice tool for making sure you don't kick left or right. Unlike a football the soccerball will bounce and roll right back to you from the wall so you can practice over and over again.
One of the other things you have to be careful about when you kick is that you get enough lift on the ball. If you don't get enough lift, the defense can block the ball. You need more lift for shorter field goals than longer ones, because the incline angle is steeper. To get the distance, the incline angle for a long field goal isn't as steep, but you have to make sure not to make the incline angle too low or the opponent will block the ball.
As far as getting more power you are going to get that by doing leg weight excercises. You just have to be careful when you do that, because as you get stronger that can throw off your kick if you're not careful.
