Look at the autocomplete HTML attribute (on form or input tags).
<form [...] autocomplete="off"> [...] </form>
Edit:
Nowadays - from the comments - web browsers do not always respect the autocomplete tag defined behavior. This non-respect could be surrounded with a little of JavaScript code but you should think about you want to do before use this.
First, fields will be filled during the page loading and will only be emptied once the page load. Users will question why the field has been emptied.
Second, this will reset other web browser mechanisms, like the autofill of a field when you go back to the previous page.
jQuery( function()
{
$("input[autocomplete=off]").val( "" );
} );<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form method="post" action="">
<div class="form-group">
<label for="username">Username</label>
<input type="text" id="username" name="username" value="John" autocomplete="off">
</div>
<div class="form-group">
<label for="password">Password</label>
<input type="password" id="password" name="password" value="qwerty" autocomplete="off">
</div>
<input type="submit" value="Login">
</form>
autocompleteis now ignored by all but the most obsolete/outdated browsersThis is not a viable solution anymore.