you can use the >{declaration} construct in the format declaration of the table. The documentation of the array package describes its use. Take a look at the following example:
Code: Select all
\documentclass{article}
\usepackage{array}
\usepackage{longtable}
\usepackage{lipsum}
\begin{document}
\begin{longtable}{>{\raggedright\arraybackslash}p{1in}>{\centering\arraybackslash}p{1in}%
>{\raggedleft\arraybackslash}p{1in}}
\caption{a test table}\\
\lipsum[1] & \lipsum[1] & \lipsum[1]
\end{longtable}
\end{document}