Step 1: Shooting
1. Ideal shooting position is halfway between the top and bottom of the building (or area of the building). This, of course, requires a ladder, or shooting from the building across the street. Horizontal position is obviously directly opposite the middle of the building, which is often helpfully marked with a door or window. A ladder isn't necessary if you have a Tilt/Shift lens (24mm" works well in many situations), but these lenses are expensive, and aren't available for all lines of SLRs (Canon has a good one, but it's $1100+). TS lenses straighten the converging lines effect that you get if you shoot up at the building from the sidewalk across the street-- you can do a lot of the same thing in Photoshop (see editing step.)
2. Hold the camera with the image plane (back of the camera) exactly parallel to the building. This is tricky and takes some practice. On a positive note, you don't have to hold it perfectly still, because the building is happy to sit still for you.
3. Often, a picture is a bit more exciting if someone is walking by, or if there is an object to grab the eye in front of the building. In the picture illustrating this step, the lamp-post adds a little something extra. If you're going for something in the foreground, make sure to use a smallish F-stop to keep the depth of field deep (F8 or above usually works fine from across the street). This way everything will be in focus.
4. Avoid any distracting elements-- that include:
-- lampposts (almost never look good unless they are at the edges, and then only if they are distinctive)
-- cars (death to most photographs because they destroy that "what year is it?" quality, and tend to block the front of buildings)
-- strange things in the background or foreground, like wires or satelite dishes
5. A word on lighting: buildings always look best an hour before sunset or an hour after dawn, and generally look better on slightly cloudy days. Bright light, particularly in the afternoon, will cast harsh shadows that make buildings look bad. Avoid shooting at noon at all costs. Avoid any shot where you can see clearly delineated shadows, unless they really work.
Step 2: Editing
1. Crop: A good photo of a building puts the building in a prominent spot in the image (not necessarily the center, but that's where I like it), and keeps the lines straight. Your best tool in this effort is perspective crop in Photoshop. That's the normal cropping tool, but with that little checkbox for "perspective crop" in the toolbar checked. Once you check it, you can drag the four crop lines at skewed angles. The trick is to line each line up with the right side of the building (top line with top of building, left with left side of building, etc.) You also want to maintain the basic dimensions of the picture, and you can't pull the lines too far off 90 degree angles without some major distortion. It's a bit tricky, but practice makes perfect.
1a. Re-crop-- sometimes the Perspective crop screws up the dimensions of the image. If you want to, you can recrop the image to 4.5x3, 4x3, 1x1, etc.
2. Color-- here, I like to use Photoshop's curves (slight s-curve to increase contrast, or rounded middle to brighten midtones, depending on the situation.) You can also use levels, selective color, or hue/saturation, but most pros I use stick to curves.
3. Sharpen-- best to always resize to your final dimensions before sharpening (for instance, I put up 900x600 on my website, so I resize to 900x600 before I do anything else.) Then, if you can, view actual pixels (100% magnification.) Then apply your favorite sharpening method. I use unsharp mask, or lab sharpening (you can look both of those up on photo sites.)
4. Final prep: sometimes you'll want to add a bit of hue/saturation to bump up the colors a bit more, or tool slightly with the contrast, but basically you are done. Save and go home!