Austria enjoys a perfectly continental climate which means that summers can be particularly hot while winters often turn out to be very cold. The country's rather high average altitude reinforces the winter coolness and sometimes leads to very low evening temperatures even in summer. In light of this remember to take with you a few warm clothes of the fleece type, whatever the season.
It is not rare for summer days to end in a thunderstorm so also remember to take an umbrella! In winter the important snowfall means you have to come equipped: in other words prefer hiking boots to trainers!
Take note also that Austria is still steeped in a certain conservatism and that the accepted dress code to go out to the restaurant or theater is formal, especially in the country's big cities and Vienna the capital. There is probably no need to take along a suit and tie for all that, a shirt will no doubt do very nicely.
If you plan to venture into the Austrian mountains, equip yourself accordingly. Good hiking boots are necessary whilst in summer, insect and mosquito repellent sprays or creams are strongly recommended.
Lastly, do not forget your camera, for Austrian panoramas are so magnificent that you will regret not being able to share their beauty when you get back!