Store the yeast at normal fridge temps, but keep it away from the top shelf where the coldest air comes out of the vents (although its not the coldest part of the fridge). The bottom of the fridge is the coldest spot and stuff is less likely to freezer there.
Secondly, the optimum storage medium in the fridge is water. After you take a sample of yeast cake you should dilute it with water (boiled and cooled water). Then shake it up to help break up chunks. Heavy clumps of protein and hop debris will settle out first. You can then pour off the dark milk like yeast layer. Let it settle over night. Then pour off the liquid and replace with an inch or two of water. This greatly reduces any residual nutrients inthe container. Yeast will slowly try and consume nutrients in the medium if possible even at cold temps. Ultimately, they yeast stress themselves out too much doing so and it becomes difficult to revive them because the viability drops.
I've store yeast like this for 3-4 weeks with good luck. Stepping in the back up in at least a two step starter is good after prolonged storage. If less than a week you can pitch the slurry directly.