title = {The {THE} operating system},
Sometimes you find BibTeX changing the case of a single letter inappropriately. No matter: the technique can be applied to single letters, as in:
title = {Te{X}niques and tips},
If your document design specification requires a different style of capitalisation, you should acquire a bibliography style that doesn’t enforce BibTeX’s default rules. It is definitely not a good idea to enclose an entire title in braces, as in
title = {{TeXniques and tips}},
though that does ensure that the capitalisation is not changed. Your BibTeX database should be a general-purpose thing, not something tuned to the requirements of a particular document or bibliography style, or to the way you are thinking today — for example, on a future occasion, you might find yourself using a different BibTeX style with different capitalisation rules.
There’s more on the subject in the BibTeX documentation.This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=capbibtex