Make the eyes bigger: To ensure that you look the part, you can outline your eyes with some dark eyeliner so that they appear larger and as close in looks to the anime.
Learn to speak her way: You need to perfect the right way of saying her name and also be able to speak some basic Japanese phrases with the right accent and you are good to go.
Cosplay can be a lot of fun if you know some background information of the character you are playing, This will not only perfect your look but also enable you to get into the mindset of the character that you are dressing up as.
The first thing you need to do is collect as much trivia about Hatsune Miku as possible. This way you know what makes her groove and what she thinks. You too can start thinking on those lines to get into her mindset.
The next thing is to study her appearance in detail. You will see the difference between good and bad cosplay is not due to the most obvious things but rather the subtle nuances are the ones that make the most difference. It is important that you give credence to all things big and small so that you get it right.
After all you are investing yourself, your time and your money on doing this, then why not go the whole hog and study the matter well? This way you will be able to be successful at the Hatsune Miku Cosplay.