North towards Tehran
The best spot is the pay-toll 15-20km North of the city. To reach this spot, take a city bus in front of Kaveh terminal (Northern terminal) to Shahin Shahr (350 Tomans in May 2012, 0,25$). Jump out of the bus when it leaves the freeway and try to catch any ride heading North, at least to the Police checkpoint North of Hesa Airport).
From this checkpoint, it is less than 2 kilometers to the junction between the old road and the freeway, and another 500m to the pay-toll.
Note : from the pay-toll, take a lift at least until Kashan. Don't go with a driver heading only to Natanz, because he will leave you in the middle of nowhere when he will leave the freeway to Natanz (there is no entrance there).