How do you clean white cycling shorts?

[XMcShiftersonX]

I bought a pair of white Assos shorts a couple months ago and I love them. But, I was wondering if anyone has any good tips on cleaning dirt spots on them. It's not grease, just a few areas where typical dirt has collected and standard washing hasn't done the trick. I thought I read somewhere that someone used Windex on their white cycling clothes and that did the trick. I only have a natural window cleaner, and I don't know if it would work the same, or not. I would hate to ruin such an expensive and nice pair of shorts!