trigger on Account (after update) {

            String billingCity = Account.billingCity;

            billingCity = toTitleCase(billingCity);
}

//converts a given string to Title Case where the
//first letter of every word is capitalised and the rest are small
public String toTitleCase(String billingCity) {
   System.debug('billingCity ' + billingCity);
   String titlePhrase = '';
   //a set of words that should always be in lower case when in Title Case
   Set<String> forceLower = new Set<String>{'of', 'the', 'for', 'and', 'a', 'to', 'at' ,'an', 'but', 'if', 'or', 'nor'};

   if(billingCity != null && billingCity.length() > 0){
      String[] splitPhrase = billingCity.trim().split(' ');

      for(integer i = 0; i < splitPhrase.size(); i++){
          if(!forceLower.contains(splitPhrase[i].toLowerCase()) || i == 0 || i == (splitPhrase.size()-1) ) {
             titlePhrase += (splitPhrase[i].substring(0,1).toUpperCase())+(splitPhrase[i].substring(1).toLowerCase())+' ';

          } else {
               titlePhrase += splitPhrase[i].toLowerCase()+' ';

               }
          }
       }

       return titlePhrase.trim();
}