I am working on a trigger that will sum some of the values from the opportunityLineItems and update a field on the opportunity. These are custom fields that do not automatically sum and our org has already used 100% of available roll-up summary fields and there are existing integration pieces that rely on these fields. 

Currently they are updated via s-control that has to be manually envoked from a link on each record. 

I have some experience with Apex Trigger, but am hoping somebody may be able to help me get started... 

So far I have:

trigger Calculate_Products on OpportunityLineItem (after delete, after insert, after undelete, 
after update) {

	//Set the OpportunityID
	Set<id> Ids = new Set<id>();
     for (OpportunityLineItem oli : Trigger.new) {
         Ids.add(oli.OpportunityId);
        }
        
	//Query the OpportunityLineItems
	List<aggregateResult> results = [select sum(margin__c)m from opportunityLineItem where OpportunityId in :Ids];
   
    for (OpportunityLineItem oli : Trigger.new) 
    {     
   	Opportunity Opp = opptys.get(oli.OpportunityId);
    Opp.Margin_Total__c = results.get('m');
    }    
     
	update opp;
} 

 but I am not quite sure how to I can sum the margin__c field and update the opportunity total_margin__c field... or how to ensure the trigger is bulk friendly. 

Thanks in advance for any help!