Slowly add the water over a long period of time. Water has a specific heat capacity of 4.18j/g-c. Therefore if the tank is a 30 gallon tank and you are doing a 50% water change there will be close to 15gallons of water in the tank (not counting the volume displaced by gravel and decorations. 15 gallons gives 57 liters and thus 238.26 kJ of energy for every degree celsius. If you have the tank at 80 degrees F (26.7 degrees c) your total energy is 6361.5 kJoules. Then you take the room temp water (we'll just arbitrarily say it is 70 degrees F or 21.1 C) and you can figure out how much water to add to keep from drastically changing the temps. Then add 1 liter of that water and you get 58 liters of water at 6431.5kJoules, divide by 58kgm, divide by 4.18 and you get 26.53 C (79.75 F). Now it may seem as if some of the stuff was unnecessary as it cancelled out in the end but there are some more factors to consider...1 watt = 1 joule/sec. If you have a 200Watt heater you can put 200 joules of energy back into the water every second...so you find that 58 liters of 80 F (26.7C) water should have an energy content of 6465.1kJ a difference of 33.6kJ from what we have...divide that by 200(watts from the heater) and you find that if you drip one liter of water into the tank every 2 minutes and 48 seconds there will be no change in temperature!
But wait, there's more! We have not considered the radiant heat given off by the tank, conduction of heat through the glass from the water to the air on the outside and thermal conductivity of the tank (whether it is glass or acrylic makes a difference) and then finally the convection of the heat amongst the water itself...so to keep this from being too difficult I say just use 75% of your heater's output (150 watts in this case) and you find that you should have 1 liter of 70 degree F water drip into an 80 degree F tank over 3 minutes and 44 seconds to keep the temps stable.
Maybe I should just make a little javascript so you can plug in your own temps and the like and it will tell you exactly what timeline should be used while hiding all the math from you? I could even have the aquarium dimensions as a field to determine the effects of heat loss to the room (or better yet, common sizes in a drop down list).