Lots of seat cover threads on the forum.
I have two labs and the best covers I have found are called "Outlaw Saddle Blanket" in Gun Metal Grey. (Note - the trim?piping is grey). They hide the dirt really well, are extremely strong and durable, can take the "clawing" from a large dog and are washable.
Here is my supplier but I believe most of the other suppliers have the same product, maybe a different name:
Other Suppliers - Cover King http://www.coverking.com/
- Cover King has the best selection of different colours, however I do not like the fact that the "saddle blanket" portion is only in the middle part of the seating area.