I think there are a few design DON'TS that really stand out: firstly, themed rooms are generally pretty awful. Mardi gras dining room in purple, green and magenta, Gothic (as in black and vampy, not as in spires and pointed arches) anything, and my personal pet peeve -- Santa Fe Western-themed rooms in England. Bad news on so many levels. Secondly, if you invest heavily in fashiony trends or fads (think shag carpet or avocado bathroom suites) your whole home looks dated very quickly. Better, in my opinion, to stick to classics and accessorise to a theme or trend. That way, when you get tired of the trend, you can easily change the look without having to redecorate the whole house!