One simple way would be to define custom hooks within your plugin that you can hook onto. The in-built hooks system allows for you to create your own hooks and then bind onto them like normal Wordpress hooks. The Wordpress Codex has great examples and explanations of the do_action function and how you can use it to create custom hooks. A seriously overlooked feature in Wordpress by plugin and theme developers.
I am a firm believer that the hooks system is all you need to develop plugins that can be extended by other plugins, but as I said seriously overlooked by 90% of all Wordpress developers.
See below for an example (taken from the Wordpress Codex link provided):
# ======= Somewhere in a (mu-)plugin, theme or the core ======= #
* You can have as many arguments as you want,
* but your callback function and the add_action call need to agree in number of arguments.
* Note: `add_action` above has 2 and 'i_am_hook' accepts 2.
* You will find action hooks like these in a lot of themes & plugins and in many place @core
* @see: http://codex.wordpress.org/Plugin_API/Action_Reference
// Define the arguments for the action hook
$a = array(
'eye patch' => 'yes'
,'parrot' => true
,'wooden leg' => (int) 1
$b = 'And hook said: "I ate ice cream with peter pan."';
// Defines the action hook named 'i_am_hook'
do_action( 'i_am_hook', $a, $b );
# ======= inside for eg. your functions.php file ======= #
* Define callback function
* Inside this function you can do whatever you can imagine
* with the variables that are loaded in the do_action() call above.
function who_is_hook( $a, $b )
print_r( $a ); // `print_r` the array data inside the 1st argument
echo '<br />'.$b; // echo linebreak and value of 2nd argument
// then add it to the action hook, matching the defined number (2) of arguments in do_action
// see [http://codex.wordpress.org/Function_Reference/add_action] in the Codex
// add_action( $tag, $function_to_add, $priority, $accepted_args );
add_action( 'i_am_hook', 'who_is_hook', 10, 2 );
# ======= output that you see in the browser ======= #
['eye patch'] => 'yes'
['parrot'] => true
['wooden leg'] => 1
And hook said: "I ate ice cream with peter pan."