I think a great tip is to look in the real world for inspiration. I'll tell a little bit of a story about my FTB Beta pack world to illustrate my point.
I firstly build a little castle/keep, based on just about every medieval castle that I got dragged to as a child, this became my home, where I mostly kept vanilla stuff.
Then decided to build a mage tower, which was a tall circular tower made using stoneslabs, broken up with windows on the corners and then about half up built a sort of helipad like thing coming out that I originally had a nether portal on, but eventually replaced this with a Mystcraft portal. I used this for all my magicy related things.
The problem then arose of what to build for my technical stuff, I spent ages pondering what it could be one evening. Then the next day on my way into work I walked past a newly built block of flats, in a sort of C shape. I then went on to use this for my inspiration for my build, right down to the very windows. Only change I made was replaced one side of the C with 3 huge chimneys.
Another real life inspiration I often steal is a project here in England called the Eden project, which are basically big glass domes. I often build these domes to house farms and the such.
Oh and another tip that I do is sketch my design on a piece of paper, especially if I can't jump straight on the computer once inspiration struck. I actually did this with my factory thing up above (spent the morning at work sketching my factory

). Then when you have your sketch you can pre-build it, as suggested above, on a flat creative world to see if it actually works in Minecraft.
Oh and two more quick tips, upside and corner staircases. Utter God-send!