In the constitution of 1949 Germany ensured that chancellors could not endow themselves with additional powers, eliminating the chance of a repeat of Hitler's 1933 episode.
What were these steps?
I can offer some points of the constitution that I remebember beeing implemented esp. as a safeguard:
Additionally, some of the individual states have articles in their respective constitutions that call for the state to appropriate key industries (coal, steel) - this, because everyone including conservatives thought back then that de-monopolization would be an important part of preventing a new rise of something like tha Nazi-party.
While not a safeguard against a new dictatorship, it is often stated that article 16 was written in response to Nazi-fascism - the right to political asylum, as a lot of people had cause to flee from germany. This practically abolished in the early 90ties.
Also, it is hard but possible to illegalize a political party, in such a case it must be proven that said party is aggresivly fighting against the constitution.
Note on the other hand, that in Germany it is unconstitutional to deliver a German citizen to the law enforcement of another country. This protects Nazi War-criminals from prosecution to this day.
There are several safeguards against the Chancellor - or any other part of the government - acquiring too great powers.
mart already mentioned Article 1, ang Gangnus mentioned the direct applicability, namely, Article 3. The core meaning of the first articles is:
In the following articles, many rights are spelled out which make it very difficult to install a dictatorship (personal freedom, no discrimination because of race, gender, beliefs etc., freedom of speech, right of assembly, privacy of personal communication, freedom of movement, no forced labor, protection of one's home and property, citizenship can't be revoked).
These basic rights are protected by the Ewigkeitsklausel ("Eternity clause"), which means they cannot be altered or suspended. A good overview on that can be found on Wikipedia: Eternity clause (Germany)
Other parts of the constitution spell out the power-sharing between the central government (Bund) and the federal states (Länder). The states can block many federal laws.
So, the Grundgesetz (the constitution) seems pretty solid and safe in this regard.
However, the Grundgesetz ("Basic law") was originally only meant as a temporary solution. Thus, it includes this:
This article would allow to adpot a new constitution without any of the safeguards if the German people could be brain-washed enough to "freely" adopt it.
The German Grundgesetz can be read in English here.
I think the main piece of news was the direct applicability of the constitution. Especially human rights.
But really, we can't say how constitution or some laws could not endure, but even merely influence the possibility of dictatorship in Germany in 1945-1989. Because only in 1989 did Germany become a sovereign state. Before that it was under strict control.
The only true defence or safeguard against any sort of authoritarianism is the vigilance of the citizenry: the refusal of the citizenry to either advocate, or submit to, arbitrary measures.
Great Britain has no constitution, and the U.S.S.R. had a wonderful constitution - the non-existence of one and the existence of the other had absolutely no effect on the actual wielding of power, and the use or abuse of power by the state..
A people always gets the government it deserves - sooner rather than later.
As such, a constitution either written or unwritten has no inherent enforceability beyond the willingness of the citizenry to risk life, limb, and freedom to enforce it.