For going to Santiago it's better to get to the highway entrance, which is quite far. Don't expect to get a ride on the way there.
On the contrary, Miriam quickly got a ride on Calle Larga all the way to Santiago even though there was not a lot of space to pull over. It's Chile after all.
Trucks heading to Argentina will stop for their customs clearance at a huge facility on the eastern outskirts of Los Andes, in San Miguel, where Highways 57 and 60 meet. It is around 4.5 km from the centre of Los Andes. This is also where all traffic coming from Santiago and not going through Los Andes will pass. There are often trucks parked on the side of the road you can ask, in addition to those driving out of the facility.
From there east along Highway 60 for some distance there are not many good spots to hitch. The road is flanked by sidewalks and the traffic is going fast, with no stop signs or speed bumps to slow them down. The area is rural-esque, with homes lining the road for still some way to the east, generating lots of very local traffic that will get in the way. Your best bet if you continue this way may be where train tracks cross the road a couple of hundred meters from the junction with Highway 57, as most traffic will slow down here and there is a spot where cars can pull off the road.
The interchange where Highways 57 and 60 meet is very large, with two overpasses and an T-junction. Coming from Los Andes you will pass under one overpass (which continues northwest as the northern bypass of Los Andes) and then the T-junction with Highway 57 to the right. The faster route to the customs facility is taking the overpass to the right (southeast), though both will get you there (turning right at the T-junction you then turn left where it meets the second overpass). Bypassing Los Andes on Highway 57 from Santiago you will pass the customs facility on your right before the interchange.