Mushrooms should be stored in the refrigerator in a breathable container. A paper bag works great. Mushrooms stored in this manner should last at least 4-5 days.
For the record, the top of the fridge is one of the worst places to store foods. Depending on the model it can actually be a few degrees higher than ambient temperature, thus accelerating spoilage.
One suggestion is to buy dried mushrooms and rehydrate them when you need them.
As for freezing, as you probably know if you freeze them and thaw directly without any intervention you will probably get brown, mushy mushrooms. There are a few ways to avoid this.
If you blanch the mushrooms first in some boiling water, then put them in an ice bath, that should stop the enzymes that cause the browning and also help with the texture some. This method should also retain the most flavor of any of the preservation methods.
As a side note, I highly recommend going from freezer to cooking directly without thawing - this should reduce the amount of mushiness you will get.
I find that slicing and sauteing the mushrooms first in some oil or butter, then freezing them, works great for most uses, since sauteed is how I usually prefer my mushrooms anyway. I just take them out of the freezer and throw them directly into the meal.
In my house the most common method is as I first mentioned, we buy dried portabellos etc and just rehydrate them into our meals. However blanching and then freezing is often better for texture and taste, it just takes a little more time.
There are some great mushroom preservation methods available on this site.
Best Answer
Already? They were probably bad last week :-). I know mushrooms often come wrapped in plastic at the store, but they'll last much longer if you take them out of the plastic. If they're discolored (and probably soft), don't eat them.