Question: I have a log In page with username and password fields. I would like to know the best place to put the log-in button below them. Should it be left-aligned, centered, right-aligned or fill the entire width of the parent?

Is there a industry standard or best practice for Android Log In Buttons?

Answer: 
An idea would be to align them below the edit text views that will be completed. That's the most elegant way to do it.

You can try something like [this](http://info.ticketleap.com/Portals/117802/images/media/android-login.jpg) 

Or maybe something like [this one](http://farm3.static.flickr.com/2649/3925689032_c7f7151c33.jpg)
The basic layout for login activity that I stumble upon everywhere in a books looks like this:

```
<TextView android:id="@+id/userNameLbl" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:text="Username: " 
    android:layout_alignParentTop="true" /> 

<EditText android:id="@+id/userNameText" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/userNameLbl" /> 

<TextView android:id="@+id/pwdLbl" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/userNameText" 
    android:text="Password: " /> 

<EditText android:id="@+id/pwdText" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/pwdLbl" /> 

<Button android:id="@+id/btn" android:onClick="doClick"
       android:layout_width="fill_parent"  android:layout_height="wrap_content" 
       android:text="Login" /> 

```

So it's pretty much similar to what @Arkde tried to show you in the second link.

Hope this helps.