Newsletter Sign-up Form: Horizontal & Centered

I need to make the newsletter sign-up form layout horizontally and centered. I figured out horizontal with help from forums, but am stuck on centering. Here’s what I did so far:

.avia-mailchimp-form .form_element > * {
text-align: center;
}
/* Custom mailchimp form*/

.avia_ajax_form p {
width:auto!important;
}
.avia_ajax_form .first_form {
clear: none!important;
}
.modified_width .button {
margin-top: 0px;
}

#top .avia_ajax_form .text_input, #top .avia_ajax_form .select, #top .avia_ajax_form .text_area {
width: 100%;
margin-bottom: 0;
display: inline;
min-width: 50px;
padding: 10px 50px 10px 10px;
border-radius: 2px;
}
.modified_width .button {
width: 100%;
padding: 10px 12px 10px;
min-width: 0;
}