Remove the head bolts one at a time, put sealer on them and retorque to 95 ft/lb. Follow the tightening sequence below.
I use Permatex p/n 80019 Aviation Form-A-Gasket No. 3 Sealant, comes in a small bottle w/an attached brush. Costs $3-$4. Permatex #2 that comes in a tube is OK, too. Other guys use teflon sealers, etc. and report good results.