The most common advice is to blow DOWN if the fan is mounted on top of the heatsink and to blow INTO the fins if the fan is mounted on the side.
For a side mounted fan, try to mount the fan/heatsink combination so the fan blows both INTO the fins and TOWARD the rear of the case. You should be able to either rotate the heatsink 180 degrees or simply put the fan on the side facing the case front so it blows through the fins toward the rear.
If you have a fan on the back side of the case, it should suck hot air OUT. If you have a fan on the front side, it should normally suck cool air IN.