SelectPriceBook Trigger

trigger SelectPriceBook on Opportunity ( before insert, before update ) {

    map<String,Id> mapPriceBooks = new map<String,Id>();

    for( PriceBook2 sPricebook : [SELECT Id, Name FROM PriceBook2] ) {
        mapPriceBooks.put( sPricebook.Name, sPriceBook.Id );
    }

    for( Opportunity opp : Trigger.new ) {
        // Change Price Book

        // On create
        if( trigger.isInsert ) {

            // New York
            if( opp.Campus__c == 'NYC' )
                opp.Pricebook2Id = mapPriceBooks.get( PB_NYC ); // returns the correct Pricebook's ID

            // Atlanta
            if( opp.Campus__c == 'ATL' )
                opp.Pricebook2Id = mapPriceBooks.get( PB_ATL ); // returns the correct Pricebook's ID

        }
        else if ( trigger.isUpdate ) {  // On delete

            // Get old value
            Opportunity beforeUpdate = Trigger.oldMap.get( opp.Id );

            // Check old value
            if(opp.Campus__c != beforeUpdate.Campus__c) {

                // New York
                if( opp.Campus__c == 'NYC' )
                    opp.Pricebook2Id = mapPriceBooks.get( PB_NYC ); // returns the correct Pricebook's ID

                // Atlanta
                if( opp.Campus__c == 'ATL' )
                    opp.Pricebook2Id = mapPriceBooks.get( PB_ATL ); // returns the correct Pricebook's ID


            }
        }

    }

}