It is not easy. First, Zagreb is a big town, so to get to the highway, the bus takes 40 minutes. Also, since the relationships are not really good between Serbia and Croatia, some people might not want you to cross the boarder in their car.
I took the bus 276 towards Dumovec, where I got off. It's just next to the motorway, but there's no good spots for hitch-hiking. The toll is 10 km away, and there's not petrol stations. Anyway, since I had gone that far away, I had to try.
Fortunately the road is under construction works, so the traffic was slow, and a truck picked me up in the middle of the motorway just after 5 minutes waiting. He dropped me off in the toll, where it took me another 5 minutes to catch the next car. I went 40 km further, but it was a small town, without traffic, and once again, I was going to need to get a ride in the middle of the motorway. The traffic was fluent now, though.
But once again, I was lucky enough that a truck stopped after 2 minutes waiting. I went with those two guys until they found a big service station. A really good place to take a car.
However, the apparently easiest place, was the worst to get a ride. I waited for almost one hour. In that time, I made friends with some truck drivers who were resting, and they gave me some food, coca-cola and offered me their trucks to sleep a nap if I was tired. But I had to keep going if I wanted to get to Belgrade before night.
Finally, a guy who didn't speak any single word of English gave me a lift to the halfway to Belgrade.
It was also a petrol station, but again it was difficult. I spoke with a truck driver, and I waited with him for 30 minutes, until he was able to keep going.
He offered me a ride to the boarder, but he stopped anytime he saw a truck, and asked if anybody wanted to take me to Belgrade.
Before the boarder, it was the toll pay point. I left the truck, and he make sure the toll guy would tell all the cars to pick the nice Spanish guy up.
I was lucky this time, and 5 minutes after the truck left, this Italian guy, who was going to Belgrade, picked me up. Finally, after 5 rides of no more than 60 km each, I was going to get to Belgrade.
But I was still in Croatia. I had to cross the boarder, and I didn't have my passport. That made the driver feel nervous, and I wasn't really confident either. But my ID was enough, and I stepped on Serbian land for the first time. Serbia is the 28th country I've visited!
It took me nearly 8 hours to cover those 400 km. I waited for so long sometimes, it was too hot, the road wasn't nice, I didn't have any food. But I had a perfect day. I met such a wonderful people. They gave me food, drinks, desserts. One of them even forced me to call back home and tell them I had arrived to Serbia.
I guess it wasn't the perfect trip, but I felt it was, and that makes everything easier.