Just an easier format for code from this SO question (not mine): http://stackoverflow.com/questions/25311736.

gistfile1.m

- (void) addTasksToSessionWithTaskObject:(Task*)taskObject withSessionInitialisationNeeded:(BOOL) needed{

    NSString *filePath = [[NSBundle mainBundle] pathForResource:pathForResourceFile ofType:resourceFileType];
    S3PutObjectRequest *putObjectRequest = [[S3PutObjectRequest alloc] initWithKey:targetFileKey
                                                                          inBucket:_bucketname];
    putObjectRequest.cannedACL = [S3CannedACL publicReadWrite];
    putObjectRequest.filename = filePath;
    putObjectRequest.contentType = [resourceFileType isEqualToString:@"MOV"] ? @"movie/mov" : @"image/jpg";
    putObjectRequest.endpoint = @"http://s3.amazonaws.com";
    putObjectRequest.contentLength=[[[NSFileManager defaultManager]
                                     attributesOfItemAtPath:filePath error:nil] fileSize];
    putObjectRequest.delegate = self;
    [putObjectRequest configureURLRequest];
    NSMutableURLRequest *request = [s3Client signS3Request:putObjectRequest];
    NSMutableURLRequest *request2 = [[NSMutableURLRequest alloc]initWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://s3.amazonaws.com/UploadTest/%@",taskObject.fileKey]]];
    
    [request2 setHTTPMethod:request.HTTPMethod];
    [request2 setAllHTTPHeaderFields:[request allHTTPHeaderFields]];
    
    if(needed) {
        
            sharedSession = [self backgroundSession];
            [sharedSession setAttemptsToRecreateUploadTasksForBackgroundSessions:YES];
            sharedSession.securityPolicy = securityPolicy;
            
            
            [sharedSession setSessionDidReceiveAuthenticationChallengeBlock:^NSURLSessionAuthChallengeDisposition (NSURLSession *session, NSURLAuthenticationChallenge *challenge, NSURLCredential * __autoreleasing *credential) {
                NSLog(@"%@ %@ %ld", challenge.protectionSpace,challenge.proposedCredential,(long)challenge.previousFailureCount);
                return NSURLSessionAuthChallengePerformDefaultHandling;
            }];
            
       
            [sharedSession setTaskDidCompleteBlock:^(NSURLSession *sessionI, NSURLSessionTask *task, NSError *error) {
                
                dispatch_async(dispatch_get_main_queue(), ^{
                    
                    __block UIBackgroundTaskIdentifier bgTaskI = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
                        [[UIApplication sharedApplication] endBackgroundTask:bgTaskI];
                    }];
                    
                    if([currentlyActiveTaskIdArray containsObject:@([task taskIdentifier])]){
                        [currentlyActiveTaskIdArray removeObject:@([task taskIdentifier])];
                    }
                   
                    
                    if(currentlyActiveTaskIdArray.count < LOWER_SLAB_FOR_TASKS){
                        [self LogMessage:[NSString stringWithFormat: @"Lower Slab For Task Reached-ADDING MORE TASKS"]];
                        [self initiateS3UploadForSetOfTasksIsItBeginningOfUpload:NO];
                    }
                    
                    [[UIApplication sharedApplication] endBackgroundTask:bgTaskI];
                });
                
            }];
            
        
            [sharedSession setDidFinishEventsForBackgroundURLSessionBlock:^(NSURLSession *sessionI) {
                 dispatch_async(dispatch_get_main_queue(), ^{
                
                AppDelegate * delegate =(AppDelegate *)[[UIApplication sharedApplication] delegate];
                if(delegate.backgroundSessionCompletionHandler){
                    void (^handler)() = delegate.backgroundSessionCompletionHandler;
                    NSLog(@"App going to be put back to sleep");
                    handler();
                }
                 });
                
            }];
        }
    
    NSURLSessionUploadTask *task = [sharedSession uploadTaskWithRequest:request2 fromFile:taskObject.fileURL progress:nil completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
            NSLog(@"Complted Task with %@ %@ %@", error,responseObject,response);
        }];
     task.taskDescription= taskObject.fileURL;
     [currentlyActiveTaskIdArray addObject:@([task taskIdentifier])];
    }

// Heres my code to add more tasks

- (void)initiateS3UploadForSetOfTasksIsItBeginningOfUpload:(BOOL)beginning{
 
    int i=0;
    for(Task *eachTaskObject in tasksArray){
        if(i < numberOfTasksTobeAdded){
            
            [self addTasksToSessionWithTaskObject:eachTaskObject WithSessionInitialisationNeeded:NO];
            i++;
        }
    
    }
}