Something that has helped me tremendously has been going to architecture design sites and shamelessly pilfering floor plans. Most come available with a picture or two of the building, or at least an artist rendition of the finished product. These have served as a basic paint by number to get me familiar with how to make things look less primitive (my work, not necessarily yours). Once I got the feel for it, I began experimenting with pallette to make things fit my world better without looking too gaudy. After that, I began to feel more at ease with architectural changes (mostly to better fit the landscape, but other examples exist).
Sometimes you can get plans in metric (particularly easy from European design firms) which make the transition to Minecraft nice and easy. Otherwise. I just ballpark the dimensions into something workable and adjust as I go.