Choose pieces that age well.
Chances are, you will still have them in ten years time, so choose something that will fit in with your lifestyle then. While it may seem like a good idea to splash out on something that defines your identity now, you might be a little different then. Remember to also choose good quality for the things you want to last well too. It's worth splashing out on a good sofa and mattress. By splash out, we mean patiently hunt down the best sale.
For style longevity, you can always trust the period furniture. Think tradition period style such as the classic Louis VI chairs, these won't date. The Philippe Starck ghost chairs are a great example of this. Mid century is also a beautiful example that transcends time, just think of the show Mad Men