Magento: how to add new fields to customer account

First create xml file in etc/module named as: Bd_Customercustomfield
<?xml version="1.0"?>
<config>
  <modules>
    <Bd_Customercustomfield>
      <active>true</active>
      <codePool>local</codePool>
      <version>0.1.0</version>
    </Bd_Customercustomfield>
  </modules>
</config>

Then create config file of your module with following code
<?xml version="1.0"?>
<config>
  <modules>
    <Bd_Customercustomfield>
      <version>0.1.0</version>
    </Bd_Customercustomfield>
  </modules>
  <global>
    <helpers>
      <customercustomfield>
        <class>Bd_Customercustomfield_Helper</class>
      </customercustomfield>
    </helpers>
<models>
 <customercustomfield>
<class>Bd_Customercustomfield_Model</class>
<resourceModel>customercustomfield_mysql4</resourceModel>
 </customercustomfield>
</models>
<resources>
 <customerattribute_setup>
<setup>
 <module>Bd_Customercustomfield</module>
 <class>Mage_Customer_Model_Entity_Setup</class>
</setup>
<connection>
 <use>core_setup</use>
</connection>
 </customerattribute_setup>
 <customerattribute_write>
<connection>
 <use>core_write</use>
</connection>
 </customerattribute_write>
 <customerattribute_read>
<connection>
 <use>core_read</use>
</connection>
 </customerattribute_read>
</resources>
  </global>
</config>

At last create a mysql file mysql4-install-0.1.0.php with following code
<?php
$installer = $this;
$installer->startSetup();


$installer->addAttribute("customer", "customer_mobile",  array(
    "type"     => "varchar",
    "backend"  => "",
    "label"    => "Mobile Number",
    "input"    => "text",
    "source"   => "",
    "visible"  => true,
    "required" => true,
    "default" => "",
    "frontend" => "",
    "unique"     => false,
    "note"       => "Customer Mobile Number"

));

        $attribute   = Mage::getSingleton("eav/config")->getAttribute("customer", "customer_mobile");

       
$used_in_forms=array();

$used_in_forms[]="adminhtml_customer";
$used_in_forms[]="checkout_register";
$used_in_forms[]="customer_account_create";
$used_in_forms[]="customer_account_edit";
        $attribute->setData("used_in_forms", $used_in_forms)
->setData("is_used_for_customer_segment", true)
->setData("is_system", 0)
->setData("is_user_defined", 1)
->setData("is_visible", 1)
->setData("sort_order", 100)
;
        $attribute->save();
$installer->endSetup();
?>