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).
Note #2 : it is possible that the bus driver to shahin shahr won't let you out at the highway, even after you insist. if this happens, continue with the bus until the city, where you will see a sign leading to the highway, it is about 1km away.
South towards Shiraz
Take a city bus to Sofeh Bus Terminal, in the south of the city. As of November 2012, they are building a metro that will also stop there, in a year or two. get past the hawkers to the other side of the highway, and walk down past all the taxis and buses on the side of the highway. many people will try to get you to take a taxi or a bus, but eventually you should find somebody going south, at least to the police checkpoint, which is about 10-ish km away? Not an amazing spot, but persist a little and you will be rewarded.