I'm hoping to take a trip round South West China - Yunnan and Sichuan mostly - for a few months.
Does anyone have any hints about where or when to go?

Fall (i.e. a bit after the rainy season starting in late September) is generally a good time if you're up for a some nip in the air. This way you can avoid things like landslides in Tiger Leaping Gorge, rain-wrecked roads, and summer tourists. If you can avoid major tourist towns (Dali, Lijiang, Zhongdian) during National Holiday week(first week of October), then the rest of the time should be lovely. If you're going Sichuan to Yunnan, there's a back way bus route from Kangding in Sichuan province to Zhongdian in Yunnan. Take a few days and enjoy the towns along the way because they're developing and changing fast! Chengdu to Kangding is an easy 6-7 hour bus ride on a major road that was completed a bit over a year ago. A lot of Chinese people claim that Zhongdian is at its most beautiful in October, when the wolf's tongue (an invasive which attracts tourists but is toxic to yaks) turns red in all the fields. But I think it's lovely in winter and spring too, once you get outside the city. Scenery is remarkable.

Thanks calyx. Sounds Great... especially the wolf's tongue in Zhongdian.

oops actually it's called wolf-poison (langdu 狼毒) and it's a "domestic invasive." Enjoy.