I'm just about to do a water change and was wondering what's the best way to get water up to the right temperature to put it back in to my tank?
One way to do this is to get an extra aquarium heater and a large container.
Set the heater to the same temperature as your tank and let the water slowly heat up overnight.
By doing it this way, you remove some of the chlorine in your water and the amount of other dissolved gases in the water will be reduced too if they are present.
You will need to mix the water in the container before you use it (the coldest water will be on the bottom of your container and warmest water on top).
You can add chlorine remover and other additives to the container before you use the water in your tank.
This very much depends on how you do your water changes. For me, I make RO water and store it outside (which means it's cold). Then I add hot tap water and a dechlorinator solution to remove the chlorine / chloramine and any heavy metals from the water.
This is probably not the most energy efficient way to do it but it allows me granular control over the temperature. It also means I don't have to prepare anything in advance (apart from the hot water) and I don't have to have another heater on hand.
You can use the same concept if you just use tap water. Start with a bucket of cold and heat it up with warm. It's really that simple. Depending on the size of your tank and the percentage of water you are replacing you might not need to worry about tenths of a degree difference in water. I'd definitely suggest that the closer you can get to the correct temperature the better but if you're replacing 10% in a 300 litre tank, the difference will probably not be that noticeable if you put water in that's a few degrees south of where it should be.
This method is based on the assumption that you've got access to hot water already. If you haven't, you're better off using TrondHansen's method instead.