Tardis,Iron X and WD40 will all work well,although a clay bar will remove it too,its a bit too harsh and will wreck the bar all to easily imho. Thin it down 1st with a spray,THEN clay it to remove every last bit.
+1 for all of the above. Clay bar will do the job but its hard work and you'll wreck the bar, tardis is the dogs bollocks for getting tar off but I don't think its widely avaliable (you would need to get it from a detailing specialist or somewhere online I think). WD40 does the job. If you can put up with the smell on your hands for a day or two after, petrol is about the best thing, just wipe it on let it soak for a while then wipe it off, just don't be smoking any fags when doing it!!