Adding Next/Previous Links

edit single.php – the link tags are there already:

<div id="nav-above" class="navigation">
					<div class="nav-previous"><?php previous_post_link( '%link', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'twentyten' ) . '</span> %title' ); ?></div>
					<div class="nav-next"><?php next_post_link( '%link', '%title <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'twentyten' ) . '</span>' ); ?></div>
				</div><!-- #nav-above -->

(and same thing further down: <div id="nav-below" class="navigation">)

then use the info from the docu:

http://codex.wordpress.org/Function_Reference/next_post_link
http://codex.wordpress.org/Function_Reference/previous_post_link

see the ‘in_same_cat’ parameter