To maximize your download speed, simply do nothing else but download. If you surf the web, or check your email, or anything else internet-related, you're taking bandwidth away from your downloading.
Just download the file and do nothing else, and your maximum bandwidth will be used for the download. If you find that the download speed is not near the maximum bandwidth of your connection, it is highly probable that the place you're downloading from is not sending the data to you fast enough to saturate your connection, which you can do nothing about.