What is the proper way to cite a legal document?

Be aware that different countries have different systems of legal citation. The jurabib package that Martin mentions implements the legal citation style commonly used in Germany. Will Hardy has implemented The Australian Guide to Legal Citation (AGLC) for biblatex.

The most widely used legal citation style in the United States is The Bluebook: A Uniform System of Citation, published by the Columbia, Harvard, UPenn, and Yale law reviews. There have been some preliminary efforts to implement the necessary functionality in (La)TeX to implement the Bluebook style--notably Frank Bennett's initial effort, LexiTeX (circa 1993), and his subsequent effort, Camel (circa 1995)--but it seems that, as of about a month ago, there still is no satisfactory implementation of the Bluebook for (La)TeX.

The jurabib package was written to cite legal and similar documents. But I think I heard once that it is now deprecated ...

Citing any kind of »legislative act« is not trivial, as the OP seems to assume. To do it correctly, you have to start with your source: Who wrote where and when exactly which words? Is it beyond doubt that your source was wrong? Has the wording been altered at a later stage? Which »version« are you citing?

In Germany there is one source, which is binding (»Bundesgesetzblatt« = federal law gazette) for the Republic. But it is really hard e.g. to follow one Paragraph through the legislation of the last 112 years, since the code of civil law came into existence.

No, citing laws, decrees and such depends on many factors. There is no standard answer.