There are a variety of tank siphons available commercially that you can use to do both jobs at once, and they're available at any store that sells aquarium supplies (including PetSmart and PetCo). The simplest kind is a gravity siphon/gravel cleaner -- once you get it 'pumping', you stick the siphon mouth in the gravel and it sucks up the waste along with the water. The water ends up in a bucket or pail at a lower level than the tank. They're pretty easy to use, and if you have a bucket that holds about 25% of the volume of your tank, just stop when it's near full.
http://www.drsfostersmith.com/Produc...04+2032+113164 is one example of this type, though there are many different variations on the theme.
Once you take the water out, dump the bucket (on plants outside is good -- there's some nice nutrients in the water

), fill it up, add water conditioner, and refill the tank. Or use a 1 gallon water jug to do the same (though it'll take a bit longer). It helps if you try to get the water you're putting back into the tank near the same temperature as the water in the tank. I use an instant-read cooking thermometer to get the temp of the water coming out of the faucet right before filling my water jugs.
Oh, and keep any buckets and/or jugs you use reserved ONLY for aquarium use, to make sure no unwanted chemicals find their way into them.
If you have a larger tank and/or a nearby water source, the Python system (
http://www.drsfostersmith.com/Produc...04+2032+113164) is a popular choice. It attaches to the faucet -- twist the collar one way with the water running, and it creates a suction. Twist it the other way, and it's now a hose to fill the tank. You can just add the water conditioner into the tank first, dribble it into the stream from the python, or have it ready and immediately add it after you've finished filing the tank. The better ones (Prime, NovAqua+) work fast enough that your fish shouldn't even notice any chlorine/chloramines in the water, let alone be affected much by them.
I only have a 10 gallon tank right now, so I use a siphon and a 2.9 gallon bucket to do water changes and cleanings -- a Python would probably be overkill for me. But when I get a bigger tank (only a matter of time, I imagine), a Python is one of the first things I plan to buy. I don't relish the thought of carrying 20-30-40 gallons or more of water 3-5 gallons at a time.
