Assuming you already know how to wash yeast, here's what I do:
Once all the sediment is gone I usually have enough yeast/water solution to fill 2 - 1/2 liter bottles.
I place them in the fridge with aluminum foil over the top of the bottles.
Once the yeast falls out I pour off most of the water from one bottle and shake (to mix) the the remaining water/yeast and pour it into a sanitized baby food jar (we have a fairly new greatgrandson) and put the lid on it.
I repeat the process for the second 1/2 liter bottle and label them with the yeast style, date, and which batch I got it from and/or which batches I've already used it in.
Now I have 2 yeasts that I can reuse to make starters with. The jars take up less room than a bottle when stacked.
One vial of yeast costs between $5.95 and $7.95 If I reuse it only once my cost is cut in half. Eventually I can reuse it so the cost is under a $1.00 for each batch.
I usually only use dry yeast once since it is already under a $1.00 although I have some that's about $1.45.
One benefit to having more yeast around than you can use is you can make a starter whenever you want and don't have to make a special trip to the local HBS.
I've woken up in the middle of the night and decided I wanted to brew something the following day. This way I can make a starter at 2 AM so I could brew the next evening after work.
Right now I have 9 vials of yeast in my fridge as well as a smak-pak, 7 baby food jars of assorted yeasts and about a dozen dry packets
...and yes, if I get a hair up my butt to make a certain brew I may not even have the yeast I need for it...it happens sometimes.
I buy my hops by the pound and my malt in 55# boxes.
If I'm going to drink I'm going to be prepared for it.
I know I may or may not sway your decision to save your yeast and in effect save you $$$ in the process, but my explanation may help someone else out there who has more desire to brew than $$$ for (reusable) supplies.
Later...