Don't worry @surlybadger, ALL of us are still learning new things when it comes to building. My main tips are:
1) If you see somebody do something neat in a build that you like, download the build and take a good look at it to see how they did it. You can learn so many cool building and decorating tricks from other players. Keep it in your toolbox for when you have an opportunity to use it. I learned stuff from facing cabinets backwards to enclosing tubs with half walls from seeing other people do it.
2) Keep building. Like with anything else, you get better with practice; nobody was great at it at the beginning. Just keep doing it and trying new things and eventually you'll get better and better.
3) Symmetry is not usually good. I know I have a tendency to want to make a symmetrical building, but I've found that symmetrical builds can be boring, For visual interest, keep some elements the same (for continuity) like the same type of windows but use different sizes, numbers of windows or positioning or use a different color or wall texture to highlight the eye up or across. You USUALLY need some variations to make the build look interesting. Take a look on the gallery and download some builds you like and see how the builder used windows, texture, color, etc. to accent parts of the building and where it draws your eye and also see how they used asymmetry to make the build interesting.
4) Finally, don't be afraid to experiment. This is game and supposed to be fun! Try different things and see how they work. It's a terrific way to learn!
Good luck on you building journey! Hope to see your works posted in the forum!