Some more small tips:
Contrast: use opposites to make some detail and differences. You don't want everything blending together. This applies to a lot of things. Stick something small, next to something big. Black next to white. Stone next to wood. Etc. This will help break up your blockiness as well.
Minor details: These can make all the difference. Adding a chandelier to a large main room can take it from bland to amazing pretty quickly. The tricky part is to not over-use details though.
Blending: This kind of goes against the first point, but it is just as essential. Don't let everything completely stand out. First thing you should do, is decide how your building will fit in with the current location. What type of building, and what style, would fit best with the current location?
Also, don't forget about the area. Often, big, blocky factory type building look wrong mostly because they are completely alone. Surround it with more industrial style building, surrounding those with more commercial and finally residential type structures. Remember, you aren't going to find a world class 7 story factory in the middle of the desert completely by itself. It'll have an entire city around it.
Finally: Distraction. Some buildings will naturally be a bit ugly by their nature. Build something to distract from it. For example, consider the Autostadt Kraftwerk in Wolfsburg:
http://galerie.chip.de/k/architektur/architektur/autostadt_wolfsburg/890144/
A typically boring, ugly, old and blocky factory. Add some beautiful lighting though, and voila. Just remember to keep it under control. It should still fit with everything, and not clash. Tricky, but when done right, it's amazing.
Basically, just treat it like a piece of art. Thanks to the magical Minecraft physics, you don't need to engineer anything. So, do everything from an artistic side of thinking.