It is usually foreigners who will study the American accent in a bid to improve their chances of obtaining a more top end job. But there are also those from other States and even English speaking countries that choose to develop or improve their American accent.
In the majority of cases, the reasons behind improving ones American accent is generally down to a career move. Actors, understudies or those competing for a high paying job will do whatever it takes to improve their chances and good diction is often the first thing they choose to revise and work on.
For those who have the means, hiring a voice coach is perhaps the best way to go but this will come at a cost. Of course there are other more affordable options, most of which are absolutely free. Self study options include watching videos either online or available from the local library, mixing with locals and practicing the way you talk as often as possible. Watching American movies and listening to audio tapes are also effective in helping you acquire that authentic American accent.
There are many courses that help with accent training and accent reduction and these can be done in the comfort of your own home. Online course have the flexibility option so you can literally go at your own pace. Most courses are relatively short but do require a certain amount of study. The key to really improving your American accent is to practice and show dedication.