Cheap garrison troops can help. Units with just infantry armed with Light personal weapons do a perfectly good job of reducing unrest while being cheap
To clarify on this, since it's a common misconception: INF+PWL are not magically cheaper, in build points per ton, than INF+PW or other INF-based formations. That is, 5,000 tons of INF+PWL costs the same as a 5,000-ton battalion of INF+PW, INF+CAP, INF+LAV, ...
However, the police strength calculation uses the square root of unit size times the number of units to calculate police strength. So 5,000 tons of INF+PWL provides SQRT(3) * 1,666 * 0.01 = 28.6 police strength, while 5,000 tons of INF+PW provides SQRT(5) * 1,000 * 0.01 = 22.4 police strength. Thus PWL provide the most police strength per BP even though they are not cheaper per ton to build.
Personally, I use my regular infantry regiments as colony garrisons as they are more combat-effective if caught in a defensive battle and can be "temporarily" reassigned to offensive actions unlike pure-PWL formations if more forces are needed. I am already building them so I may as well find them something to do during peace time.