If you want to take advantage of the powerful `\newtheorem`

command
without the constraint that the contents of the theorem is in a sloped
font (for example, you may want to use it to create remarks, examples, proofs,
…) then you can use the AMSLaTeX *amsthm* package
(which now supersedes the *theorem* package previously
recommended in these answers).
Alternatively, the following sets up an environment
`remark`

whose content is in the default roman font.

The\newtheorem{preremark}{Remark} \newenvironment{remark}% {\begin{preremark}\upshape}{\end{preremark}}

- amsthm.sty
- Distributed as part of amslatex
- ntheorem.sty
- ntheorem
- theorem.sty
- Distributed as part of 2etools

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=theoremfmt