Another thing to think about when developing good foils is creating a goal that is compatible for both parties. This is harder than it looks. How do you create two characters with different backgrounds who want the same thing, but don't want to beat each other up in order to attain the same thing?
Jeeves and Wooster are all about helping out Wooster's mish-mesh of friends -- Wooster because he has a sort-of sympathy for those like himself. And Jeeves, well, I think Jeeves does it because soap operas haven't been invented yet. No -- Really, Jeeves has found the perfect position: he rules the roost, influences men of power, and has enough money left to gamble. Deep down, I think Jeeves and Wooster are very similar. Helpful. Bossy. Just separated by Opportunity.
Mike and Psmith are all about adjusting to the surrounding circumstances. Mike's been booted out of his old school for bad marks. Thus losing the heart to play cricket, his passion. Psmith has been booted out of Eton and is also adjusting to a new school. His answer is to dominate the new school. He finds a willing accomplice in Mike because their goal is the same: make the best of this school year and hope to heaven that the next year will be better.
See? Two different pairs of foils and both are balanced nicely. Wodehouse does this throughout his work. I'm working through some of his short stories right now and it's a pattern Wodehouse seems to have mastered. If only all of us could hit on character balance and goals that easily!