I'm having a hard time getting my lens to perfectly align parallel to the subject. This is very important when photographing subjects such as buildings or interiors. Do I just need to practice seeing the geometry of my subject better or are there some easy tips to ease the alignment?
Have someone hold a small mirror flat against the wall/building in line with the camera and adjust until you can see the camera reflected in the mirror. If you need to be centred on the wall as well, measure the centre of the wall and place the mirror there.
Apart from that, I can only suggest zooming until the edges of the wall/building and/or floor/ceiling are near the edges of your viewfinder image and align the camera so that the perspective is equal on both sides, top and bottom.
Another possibility with interiors especially is to align the shot with natural lines parallel or perpendicular to the walls, like the lines of wooden floors, tiles, furniture. You might crouch down, or aim the camera down to align it, then stand and recompose but maintaing that alignment.
MikeW is right about the mirror, but it's not enough to see the camera reflected in the mirror. To be perfectly parallel, the centre of the lens must be in the centre of the screen. Use focus points or grid lines or whatever your camera has to indicate the screen centre.
I too have found it hard to get the sensor perfectly parallel. But if you use Lightroom 4, you dont' have to. In the "camera correction" portion of "develop" you can adjust a slider and it will correct both vertical and horizontal misalignment.
Its not as good as perfect alignment, or using a proper tilt/shift lens, but it was good enough for the photos I took when I sold my house.