Running a "cheap" DC motor in low stable RPM?
It is really difficult to drive it electronically so slow without some kind of feedback (like an opto or phono wheel). It depends a lot on your torque profile, kind of load and required speed precision.
You can simply use the usual PWM, the motor winding does the current smoothing. Use a somewhat low frequency to have more resolution if needed. But given the high tolerances of these motor you'll probably need some speed feedback to make it work reliably.
David777 also recommended a gearbox. It's expensive but reliable and you get more torque. Some feedback or tuning will be required to keep the speed stable.
For really stable speed without any kind of control you may want a synch mains powered motor; they use them to run clocks since it works exactly with the mains frequency. it's more expensive, however.