I agree with Tamie - it is always best to remove the precursor (i.e. organics) from the source water prior to chlorination to minimize the THM formation potential. If this cannot be achieved, then aeration may be helpful as mentioned by richardn209. Ensure that the aeration equipment produces the optimal bubble size capable of removing the volatile components of the total THM. We have been operating a pilot study for about 9 months that shows approximately 75 - 80% THM reduction through aeration process. It can be costly to purchase and operate the equipment, but so far appears to be highly successful in significantly reducing total THM.
If you are asking as a customer, one simple way to reduce exposure to already-produced tthms is to simply decant tap water in a vented container, such as a pitcher, overnight.
Trihalomethanes and chlorine are volatile compounds, and left to settle and exposed to no pressure, they'll volatilize out of the water. It's the same principle as aeration, just cheaper/easier/less efficient.