How to Customize the Lead Conversion Process in Salesforce.com?

trigger lead_onConvert on Lead (after update) {

/*  Create variables to store the lead values,new and old.  This will be used below to determine if the leads are being converted,  not converted yet, or has already been converted.*/

List<Lead> newLeads = trigger.new;
Map<Id, Lead> mOldLeads = trigger.oldMap;
Lead oldLead;

/* Create sets of Ids to store the records associated with the converted leads */

Set<Id> convertedAccountIds = new Set<Id>();
Set<Id> convertedContactIds = new Set<Id>();
Set<Id> convertedOpportunityIds = new Set<Id>();

/* Loop through the leads submitted through this trigger.  Populate the appropriate sets of Ids for each lead with populated values.*/

for (Lead l : newLeads) {

if (l.convertedAccountId != null) {
convertedAccountIds.add(l.convertedAccountId);
}

if (l.convertedContactId != null) {
convertedContactIds.add(l.convertedContactId);
}

if (l.convertedOpportunityId != null) {
convertedOpportunityIds.add(l.convertedOpportunityId);
}
}

/* Extract the records associated with each set of Ids populated above.  Once the data has been extracted store it in a map so it can efficiently be referenced
 in the main loop below.*/

List<Account> accounts =[SELECT Id, Name FROM Account WHERE Id IN : convertedAccountIds];

Map<Id, Account> mAccounts = new Map<Id, Account>(accounts);

List<Contact> contacts =[SELECT Id, Name FROM Contact WHERE Id IN : convertedContactIds];

Map<Id, Contact> mContacts = new Map<Id, Contact>(contacts);

List<Opportunity> opportunities =[SELECT Id, Name FROM Opportunity WHERE Id IN : convertedOpportunityIds];

Map<Id, Opportunity> mOpportunities = new Map<Id, Opportunity>(opportunities);

/* Create lists of records to be updated in bulk at the end of this trigger.  We use a separate list to ensure we aren't updating records that are  not affected by this trigger. */

List<Account> updateAccounts = new List<Account>();
List<Contact> updateContacts = new List<Contact>();
List<Opportunity> updateOpportunities = new List<Opportunity>();

/*
* MAIN LOOP
*
* Loop through all leads submitted through this trigger.
* If this trigger was executed because of a conversion
* we will populate the related records.
*/
for (Lead newLead : newLeads) {

// Get a reference to the old values associated
// with this lead.
oldLead = mOldLeads.get(newLead.Id);

// Determine if the lead was converted or not.
// If the previous status was not converted and
// the current status is converted, then this
// trigger executed because of a conversion.
if (!oldLead.IsConverted && newLead.isConverted) {

// ** CONVERSION FOUND **

Account account = mAccounts.get(newLead.convertedAccountId);
// Make appropriate updates here.
updateAccounts.add(account);

Contact contact = mContacts.get(newLead.convertedContactId);
// Make appropriate updates here.
updateContacts.add(contact);

Opportunity opportunity = mOpportunities.get(newLead.convertedOpportunityId);
// Make appropriate updates here.
updateOpportunities.add(opportunity);

}
}

/* Update the records for any update list  populated with records.*/

if (updateAccounts.size() > 0) {
update updateAccounts;
}

if (updateContacts.size() > 0) {
update updateContacts;
}

if (updateOpportunities.size() > 0) {
update updateOpportunities;
}
}