If you don't want to go the yeast washing route, just saving the trub (unless you dry hop with loose hops, not in a bag) in a jar will work. I've done this several times with excellent results. Some things to consider; if your brew that you plan to save the yeast from doesn't turn out great, *don't* save and re-use that yeast (although it sounds like you have a good handle on the process). What I do, once I've racked off the beer into the keg, is stir up the goo on the bottom of the fermenter with the leftover beer until it's liquidy enough to pour, then just pour or scoop it into sanitized mason jars. Seal them up, mark them with the date, and refrigerate. When you brew again and want to use them, take them out of the fridge at least an hour or so before you pitch to let them warm up to room temperature. Loosen the lid so any gas can escape. Before pitching, pour off the beer that will have risen to the top, then shake it up and pitch what's left in the jar. Some may disagree, but this works for me just as well as washing yeast without the hassle. As long as it isn't too old, two 16 oz jars will be more than enough for a 5 gallon batch.
So let's say you have some slurry in your fridge that is more than a month old, and you're not quite sure it will work. Well then, just make a starter with it! Pour off the "old" beer, and add the sludge to a growler with some DME dissolved in hot water that's been chilled down under 70 degrees. Apply an airlock (or if you don't have one, cover with some sanitized foil) and watch for action. I don't have a fancy stir plate, so I just give it a good swirl several times a day. Once the krausen drops and you've got a good layer of trub on the bottom, you can either refrigerate and hold it until your next brew day, or pour off the "done" beer from the top and add some fresh DME wort to keep the party going and get more viable cells.