Goran is right about staying away from the main coastline and hopping down the islands, it's a great option. Wild camping is hard to get away with, and not worth the trouble with police or locals - no vacation needs that stress.
From Istria, it's possible to take a short ferry from Brestova to Porozina on the island of Cres and start your island hopping right there. Many Warmshowers guests have departed our house in the morning and spent the night on Cres. There is a nice camping close to the port.