sanitation, sanitation, sanitation!
this is how i did it, after a bit of research and figuring what would work with my equipment on hand.
after racking the beer off the yeast cake, I added about a gallon or so of sterilized (boiled for 10 mins and cooled) water. I sloshed that around a bit, then poured it into a gallon-size jar.
I let that settle on the countertop for about an hour or so, and poured the liquid portion into 3 or 4 mason jars. be careful not to disturb that bottom layer of sediment. after that, i just labeled the jars with the type of yeast and date, then put them in the fridge.
I think i used them up in about 6 months or so, and they seemed to be fine. I should have made a starter every single time when reusing that yeast, but sometimes I got a bit lazy and fermentation lagged a bit.
no idea how much wyeast is compared to white labs, but when you can buy hops and grain in bulk, this was the last area where I could tighten the budget a little bit
good luck!