One way to prevent yellowing of exposed plastic parts is to simply spray paint them before installing them. Today's spray paints that are formulated for plastic are fantastic. if you use several light/thin coats the finish turns out amazing!

You can also paint existing parts that have already yellowed, simply mask around them and use a good primer (or paint with primer) to give the paint something to adhere to (I like the self etching primers myself [Duplicolor brand is my absolute favorite]).