How can I print out a single stylesheet or javascript link?

to print a single style:

wp_print_styles('admin-bar');

to print a single script:

wp_print_scripts('admin-bar');

note that the handle must be registered first using wp_register_style() or wp_register_script().

http://codex.wordpress.org/Function_Reference/wp_register_script
http://codex.wordpress.org/Function_Reference/wp_register_style