I use a DSLR and I very often find that when I shoot without a tripod, and with my eye to the viewfinder (as opposed to using LiveView), my photos end up slightly crooked: lines that should be horizontal or vertical are slightly slanted.
Obviously I can fix this very easily in post-processing, but it would be nicer to get it right in camera. I always try to use the viewfinder's focusing points to straighten my composition (by lining them up with a horizontal or vertical element in the scene) but still, the photos so often end up crooked!
Are there any good tips for getting better at this? Is it a posture thing? Do (some) people tend to lean slightly as they press the shutter button? I'm constantly amazed at my inability to do such a simple thing right. Help!