You can use the standalone class for this. In v0.x it used preview internally, but for v1.x it also has an alternative crop option, which works similar to the preview option/package, but avoids its issues with XeTeX.
There is now (v1.0) also a tikz class option which turns any (outer) tikzpicture into a single tight page. This avoids issues with trailing implicit paragraphs. In addition it automatically loads the tikz package.
% tikzpic.tex
\documentclass[crop,tikz]{standalone}% 'crop' is the default for v1.0, before it was 'preview'
%\usetikzlibrary{...}% tikz package already loaded by 'tikz' option
\begin{document}
\begin{tikzpicture}
\draw (0,0) -- (10,10); % ...
\end{tikzpicture}
\end{document}
Then compile it as usual with pdflatex or xelatex etc.
To include this tikzpicture into a main document load the standalone package there with the option mode=buildnew. Then use \includestandalone[<options>]{<filename>} instead of \includegraphics. This will compile all includes standalone files automatically as graphics and build these graphics if the source file is newer than the existing graphics file. This needs -shell-escape to be enabled to allow the main LaTeX run to call further LaTeX compilers.
See the standalone manual for more details.
% main.tex
% compile with `pdflatex -shell-escape main` or `xelatex -shell-escape main`
\documentclass{article}
\usepackage[mode=buildnew]{standalone}% requires -shell-escape
\usepackage{tikz}
%\usetikzlibrary{...}
\begin{document}
Lorem ipsum ...
\includestandalone[width=.8\textwidth]{tikzpic}
Lorem ipsum ...
\end{document}
previewpackage, which is not what you want - see the answer using TikZ'sexternallibrary instead.) – diabonas Oct 20 '11 at 11:26standaloneclass option which crops atikzpictureor similar but doesn't usepreview. This should also avoid issues like this. – Martin Scharrer Oct 20 '11 at 23:41