Most of it is just practicing the timing of your button presses.
If your timing is near-perfect the ball will go where you want it to.
If you are unsure of your ability to take the shot with proper timing, aiming a bit more inside the posts rather than at the extreme corners could help keep your shot on target, especially if you put curve on the ball.
Also, if you can't get the timing quite right for power, it's better to undershoot the mark rather than overshoot it.
An underpowered shot should at least stay on-target, giving your attempt some hope of finding the net.
An overpowered shot is practically guaranteed to miss.
gaming.stackexchange.com/users/885/Ian
Aim for upper corners, hold LB to hit a hard shot and practice the length you hold B for in the practice arena (press a button on the directional pad to start a free kick).
gaming.stackexchange.com/users/900/Karl
I spent an hour one day practicing free kicks in the Practice Arena and even though I have scored a few free kicks in the past, I came to the conclusion it was basically impossible.
I now set-up some sort of pass for every free kick no matter how close to the goal and have had a lot more success - even if it is a lot less sexy.
gaming.stackexchange.com/users/None/
What I do (and I am successful almost 40% of the times) is that I aim a little away from the post
and I try to bring the ball back in through direction keys after hitting the ball.
Through direction keys you can put curve on the ball after the ball is released. About power, well, it depends on your distance from goal.
If it is too far, then more power is needed.
For more power, hold the kick button longer.
gaming.stackexchange.com/users/1509/Aamir
Try a Knuckleball shot.
Basically push the analog stick towards goal and shoot.
For more chance,aim it at the corners.
gaming.stackexchange.com/users/90978/FIFAPRO12
I'm a fairly long-term fan of the Fifa series, having owned various Fifa titles over the years however if there is one thing that I just  can't  seem to get right, it's free-kicks on goal.  No matter what I seem do, they either go skyward, or are so lacking power that the goalkeeper has so much time to react that he easily saves it.  Any tips would be great!
How should I be performing free kicks in Fifa 2009?
gaming.stackexchange.com
<fifa-09>
gaming.stackexchange.com/questions/2198
gaming.stackexchange.com/users/340/Siyfion
You need to practise your timing so it's near perfect. Aim for upper corners and practise the length you hold B for. While practice and timing play an important role in getting shots on target, try aiming for the upper corners, holding LB for a hard shot, or aiming a little way from the post and pushing the analog stick towards the goal and shooting.
Users suggest more practice on timing. Users suggest particular moves in order to get the desired result.