Coming back from the Postojna caves walk towards the entrence to the highway where you have to pay the toll. You will pass supermarkets and a petrol station (if i remember correctly). On the last crossroad before entering the highway you will see a yellow sign that says 'straight - city that starts with an 'R' (i forgot the name) and Koper below it' , right - ' Koper'. Do not try to hitch a car that goes straight as this is just a local road, most of the cars do not enter the highway!!! Try to hitch on the bus stop on the 'old road' to Koper, the one marked as 'right'. If you're going to Trieste and someone offers you a ride to Sezana accept it - there are 6 buses going from Sezana to Trieste (the last one about 7 pm), a ticket costs 1 euro (hard to believe but it's true, yes, 1 Euro). We waited about 10 minutes for a car to stop.