Keeping Horizons Level
I'll admit it; one of my pet peeves is seeing photos that have the horizon line crooked. I'm not exactly sure why this is, but it could have something to do with the fact that I have suffered from this affliction more than once myself. If you have as well, here's a few tips on how to prevent it:
Buy a level. Hama and others make a small two-way spirit level that fits in the flash hotshoe. This is the surest way to line things up, but is not always convenient.
Switch to a focusing screen with a grid. For Nikon cameras with interchangeable focusing screens, the "E" screen has all the regular markings of the standard "B" screen with the addition of an etched-on checkerboard grid pattern. You can then line up the grid with the horizon line. Similar screens are made for Cannon cameras and those of other manufacturers.
If you don't have either of these, try to make sure your tripod is at eye level. If you need to lean your head over to see through the viewfinder, the job of keeping things level becomes that much harder. If you just can't reach the viewfinder even with the tripod at its maximum height, try composing the shot first with your head leaned to the left, then to the right. By averaging what things look like both ways you should be able to accomplish your objective.