I just did my first century in prep for 111 miles in August. A trainer told me to do a very basic weights workout every 5 days - Squats, rows, dead lifts and push ups. I added hammer curls and wrist flexes. It seems to make a big difference on hills (I think it's the squats). It's also not such a big or complex deal that it distracts you from riding, good for a rainy day.

Somewhere on these boards is a long and heated discussion about weight training and biking

I think Pilates is a great idea - I wish they had it at a convenient time at my Y. I do a little abs routine every morning, which I thinks helps too. I do a few pilates stretches after my rides too.