Or...you could pour a small portion of your starter into a small mason jar on brew day and store in the fridge for a week or 2 until you are ready to make another starter...
After years of successful yeast harvesting and repitching, I've recently switched to overbuilding the starter and saving pre-fermentation yeast. It's great to have a clean sample that is absolutely pure. I try to rebuild the culture every 3 months. My general method is this:
Reculture: Overbuild a starter and save yeast to reculture in about 3 months. Pitch most of the yeast into a standard-gravity batch.
Repitch 1: Pitch 1/3 of the yeast cake from the batch into a second batch.
Repitch 2: Pitch 2/3 of the yeast cake into the third batch.
Repitch 1 and 2 happen about a month apart, which is why I save the bigger portion of the yeast cake for a second repitch. If I'm brewing two more beers with that yeast at about the same time, I'll split the yeast cake into the other two batches. Once the yeast has gone through the second batch, I may choose to pitch the whole cake into a high-gravity batch, or just retire that yeast and go back to the reculture step.
Hope this makes sense! I have 5 yeast strains in my bank and try to reculture every 3 months.