Today my team configured AWS CloudTrail to deliver log files to an S3 bucket.  This required us to attach a specific S3 bucket policy to the bucket:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AWSCloudTrailAclCheck20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:GetBucketAcl",
            "Resource": "arn:aws:s3:::myBucketName"
        },
        {
            "Sid": "AWSCloudTrailWrite20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::myBucketName/[optional prefix]/AWSLogs/myAccountID/*",
            "Condition": {"StringEquals": {"s3:x-amz-acl": "bucket-owner-full-control"}}
        }
    ]
}