Well ventilated room with wave maker pointing towards the water surface or a small fan blowing on the water surface should do the trick as well. Personally, I will try these methods before I use buffers.
Another method is to replace evaporated water nightly with water mixed with kalkwasser and dripping into the tank, I prefer this method as I find it:
1) Safer (Will not cause ionic imbalance compared to other method)
2) Cost effective (Cheaper)
3) Lesser work (I need to replenish Fresh water anyway. Also, I need not worry about individual dosing of kh, ph, mg, ca)
http://www.aquarticles.com/articles/saltwa...kalkwasser.html
http://www.reefscapes.net/articles/breefcase/kalkwasser.html
http://www.simplifiedreefkeeping.com/faq/17.htm
HTH