How best to include other scripts?

I tend to make my scripts all be relative to one another. That way I can use dirname:

#!/bin/sh

my_dir="$(dirname "$0")"

"$my_dir/other_script.sh"

I know I am late to the party, but this should work no matter how you start the script and uses builtins exclusively:

DIR="${BASH_SOURCE%/*}"
if [[ ! -d "$DIR" ]]; then DIR="$PWD"; fi
. "$DIR/incl.sh"
. "$DIR/main.sh"

. (dot) command is an alias to source, $PWD is the Path for the Working Directory, BASH_SOURCE is an array variable whose members are the source filenames, ${string%substring} strips shortest match of $substring from back of $string