OK here we go, stick this inside your <Window.Resources> tag in the XAML code. (if you dont already have that tag then just add one right at the top of the XAML, just after where the Window and the XAML schema's etc are declared)
Code:
<Style x:Key="TransparentHighlightStyle" TargetType="ListBoxItem">
<Style.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Transparent"/>
</Style.Resources>
<!---Rest of your Style Triggers/Setters etc go here-->
</Style>
So now that we have defined the Style, we can use it like this when we declare a listbox:
Code:
<ListBox ItemContainerStyle="{StaticResource TransparentHighlightStyle}" />
Obviously your listbox will have more attributes than that but yeah, its the ItemContainerStyle attribute you need to set.
EDIT: Orrrr if you have blend then you can do it Dean's way
I prefer to do simple bits and bobs like this in the XAML code myself though, as you end up with less XAML code to look at usually compared to when Blend generates it. But hey, whatever works better for ya.