I usually take as much beer off as possible, add back a quart of water, swirl to mix it up, and pour into a couple of sanitized quart mason jars (throw any extra away - don't need that much), and store in the fridge.
If you use within a month, just toss one of the quart jars into the wort. No starter required. I usually split the 2 quarts into 3 pint jars; and a pint is enough for a batch.
If you leave it longer, I would recommend making a starter (which will be very active) to check the yeast is fine. The only problem I have found with keeping it up to 6 months has been autolysis, but it does not affect the yeast ability to ferment - just make a starter and off it goes; all those dead yeast cells are nutrient for the live yeast.