Before I got mine (get one as soon as the money comes to you, you WON'T be sorry) I would add plenty of salt to the ice water (like how you make ice-cream) and I would make frequent Ice/water batch changes. Also keep moving the water around the kettle moving. As the water gets warm around the sides of it, swirl more in.
If you notice the water start to get warm, drain the sink, add more ice, salt, and water, and proceed.
In the last 15 minutes of the boil, drop your brew spoon inside the kettle, and don't touch it, that way it will be sterile. Every little while stir the wort to move it against the cooler sides of the kettle. Also have a thermometer in there, either a floating one, or one hanging from the side....
After a couple of ice changes, partially cover your drain with something, but leave it partially open so water flows down SLOWLY (I could actually position my kettle to block the drain, and put a small plate under it to lift one little corner of it, OR if you have a double sink, let the water overflow into the other sink and drain, and turn the faucet on to a slow to medium trickle to keep the water moving and changing.
Hope this helps