I did the Nike 10k London run and my trainers got filthy. I was pissed off because it made me look like a bit like someone who might know what they were doing, when I am a absolute rank amateur. Anyway I asked my personal trainer about this (nowhere near as glam as it sounds) qnd she said she put hers in the washing machine (and she has a degree in sports science so MUST know about things like this!). She said low temp - ie quick wash or equivalent, preferably in a pillow case or similar, stuff with newspaper to dry and definitely no tumbling. She said it probably shortened their active life, but only by nano-seconds.