No space following macro without argument

The problem of using \naive\ or \naive{}: If you happen to forget the closing backslash/braces, you'll end with gobbled space without noticing it.

The problem of adding \xspace to a macro's definition: This may produce inconsistent spacing if the macro uses \emph. See xspace and italic correction for details.

In the thread linked above, Will Robertson suggested to use "delimited" macros (e.g. with / at the end) as an alternative. The main advantage of \naive/ is that an error message will occur if you happen to forget the closing slash.

(See also the comments to this answer.)

\documentclass{article}

\newcommand{\naive}{}% To make sure that \naive isn't already defined    
\def\naive/{na\"{\i}ve}

\begin{document}

the \naive/ approach

\end{document}

Use the package xspace. It

provides a single command that looks at what comes after it in the command stream, and decides whether to insert a space to replace one "eaten" by the TeX command decoder.

For your case it can be used as in the following:

\documentclass{article}

\usepackage{xspace}

\def\naive{na\"{\i}ve\xspace}

\begin{document}

the \naive approach

\end{document}

Use braces \naive{} or \naive\ or xspace. See the FAQ: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=xspace