I wash it by hand - had a bad experience where because it is a fairly stiff sort of cover it didn't bend very well in the machine and it 'rose up' to the top. When it spun around, it was in contact with the rim of the washing machine. Because the cover is polyester type, the friction between them resulted in quite severe damage. The car seat cover has this massive burn down one side and the machine had a melted plastic rim that catches clothes as they are pulled in and out.

So I recommend hand washing.
Thanks for the tip! I just got it out the washing machine an it seems fine - ill keep it in mind for the future though, I don't want ruined covers!