Add a one-time-use 14 day trial to a monthly recurring membership level with Paid Memberships Pro. Place this file in wp-content/plugins/pmpro-customizations/pmpro-customizations.php on your site, then active the plugin through the WP dashboard.

pmpro_customizations.php

<?php
/*
Plugin Name: PMPro Customizations
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Customizations for Paid Memberships Pro (14 day trial on level 1)
Version: .1
Author: Stranger Studios
Author URI: http://www.strangerstudios.com
*/

/*
	14 day free trial for monthly plan
	
	Assumes level 1 is a level with an initial payment and recurring payments set to the same amount.
*/
//remove the initial payment if we're doing a trial
function my_pmpro_checkout_level($level)
{
	global $current_user;
	
	if(!empty($current_user->ID))
		$trial_used = $current_user->pmpro_trial_level_used;
	else
		$trial_used = false;
		
	//check if the trial has already been used
	if(!empty($trial_used))
		return $level;
		
	if($level->id == 1)
	{
		//set initial payment to $0
		$level->initial_payment = 0;
	}
	
	return $level;
}
add_action("pmpro_checkout_level", "my_pmpro_checkout_level");

//push the subscription start date back if we're doing a trial
function my_pmpro_profile_start_date($date, $order)
{	
	global $current_user;
	
	//logged in at all?
	if(!is_user_logged_in())
		return $date;
	
	//check if the trial has already been used
	$trial_used = $current_user->pmpro_trial_level_used;
	if(!empty($trial_used))
		return $date;
	
	//add trial
	if($order->membership_id  == 1)
		$date = date("Y-m-d", strtotime("+ 14 Days")) . "T0:0:0";
 
	return $date;
}
add_filter("pmpro_profile_start_date", "my_pmpro_profile_start_date", 10, 2);
 
//update the level cost to make sense with trial
function my_pmpro_level_cost_text($cost, $level)
{
	global $current_user;
	
	//logged in at all?
	if(!is_user_logged_in())
		return $cost;
	
	//check if the trial has already been used
	$trial_used = $current_user->pmpro_trial_level_used;
	if(!empty($trial_used))
		return $cost;

	//add trial text
	if($level->id == 1)
	{		
		global $pmpro_currency_symbol;		
		$cost = "The price for membership is <strong>" . $pmpro_currency_symbol . $level->billing_amount . " per Month after your 14 day trial</strong>.";
	}
 
	return $cost;
}
add_filter("pmpro_level_cost_text", "my_pmpro_level_cost_text", 10, 2);

//record when users gain the trial
function my_pmpro_after_change_membership_level($level_id, $user_id)
{	
	if($level_id == 1)
	{	
		//add user meta to record the fact that this user has had this level before
		update_user_meta($user_id, "pmpro_trial_level_used", "1");
	}	
}
add_action("pmpro_after_change_membership_level", "my_pmpro_after_change_membership_level", 10, 2);