How to avoid the MIXED_DML_OPERATION error

public class MixedDMLFuture {
     public static void useFutureMethod() {
         // First DML operation
         Account a = new Account(Name='Acme');
         insert a;
         
         // This next operation (insert a user with a role) 
         // can't be mixed with the previous insert unless 
         // it is within a future method. 
         // Call future method to insert a user with a role.
         Util.insertUserWithRole(
             'johndoe@acme.com', 'jdoe', 
             'johndoe@acme.com', 'DOE');        
     }
 }  
 
 public class Util {
     @future
     public static void insertUserWithRole(
         String uname, String al, String em, String lname) {
 
         Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
         UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
         // Create new user with a non-null user role ID 
         User u = new User(alias = al, email=em, 
             emailencodingkey='UTF-8', lastname=lname, 
             languagelocalekey='en_US', 
             localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
             timezonesidkey='America/Los_Angeles', 
             username=uname);
         insert u;
     }
 }