The power of the Habsburgs were initially limited to areas of today's Aargau canton in Switzerland. By the 13th century they had gained several nearby domains through marriage, expanding towards Upper Alsace and Swabia. A lot of these were through fortunate extinction of other noble families.

One major step was when Rudolph IV, count of Habsburg, was elected King of the Romans, a title that may lead to coronation as Holy Roman Emperor. Rudolph came at the right time, just as the House of Hohenstaufen, who had dominated HRE emperorship from the 12th century, had died heirless and started an era where feuding German families vied for the title. Using his kingship he was able to control Austria and Styria through a mix of politicking and outright forgery as a Duke.

From here I will cite a previous post: http://www.reddit.com/r/AskHistorians/comments/30n0fw/in_the_15th18th_century_were_newer_powers_like/cpui9hy

Austria's rise, or more precisely the Habsburgs' rise, did take quite a long time. The HRE succession within the Habsburg family was not nearly as given in the beginning. The Habsburg were able to become HRE emperors due to Sigismund, elector of Brandenburg and reigning emperor, had no surviving children. Albert II of Habsburg was elected king of Germans, ordinarily to lead into election as emperor, but died before he was coronated. His son Frederick III of Habsburg was lucky enough to survive and be coronated emperor. The next in line, Maximilian I, gained the Duchy of Burgundy through marriage to Mary of Burgundy but lost his son Philip the Handsome. He had to put in a lot of political effort and bribes to ensure his grandson Charles V would be elected King of Germans, and eventually Emperor. And that, only after Charles V survived a bidding war against Francis I of France, who put himself into consideration for Emperor. It wasn't until much later that the Habsburgs were able to control election sufficiently that it became certain.

I would like to add that during the Thirty Years War, there were occasions where Habsburg control of HRE emperorship was threatened, especially given that control of HRE electorate was a key issue in the TYW. However the House of Habsburg continued to prevail.

It is important to cite the Habsburg dynasty's motto, "Leave the waging of wars to others! But you, happy Austria, marry; for the realms which Mars awards to others, Venus transfers to you."