I boil off 500ml of water in my 2 liter Erlenmeyer flask and cover it with foil immediately. I let it cool to room temp which is the same room as my sludge left over from the last brew. I sanitize my hands, the carboy, foil and everything around me (I keep starsan in a spray bottle with distilled water). I swirl the sludge up in the carboy then pour 500 ml into the flask and quickly cover with foil again. I swirl this around a bit then let it settle in the refrigerator.
On brew day, I set the flask on the counter and let it come to room temp. 30 minutes before pitching, sanitize everyting again I decant off most of the water. Sometimes I pitch the whole thing and sometimes I try to slowly pour the yeast in leaving as much trub behind as I can. By then the homebrew I have had during the brew day makes my a bit shaky and I get a little sloppy trying to run a thin thread of yeast into the fermenter.
I have had perfect luck with this method. Sometimes I keep about 1liter of sludge if I am making a bigger beer.