*coughcough* When using AF, it's generally best to use the centre AF point, then ones that cross the centre vertical or horizontal axis, then others (in order of decreasing resolution and how much light they need to accurately focus; the centre AF point is always you're best bet if you have the time to recompose the shot). If you do this, put the focus point over where you want to focus, then press the shutter button down half-way, then recompose, and push it all the way down to expose. If using a tripod, use mirror lock-up and a cable release to reduce vibrations and get the sharpest image possible.
That, or lend some trust to your eye and manually focus. Learning to manually focus accurately can be a boon, for whenever you're in situations where the AF might get confused (say if things are moving by in the foreground, and you need to focus on something distant, the AF might fly back-and-forth between the two).