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Depending on how hard you used the oil, you don't have to throw it away. High temperature is probably the hardest on oil. Frying fish usually results in oil that is a bit fishy in which case frying potatoes helps absorb some of the smell. Flour coated ingredients usually results in a lot of sludge, and that will hasten the break down of the oil.

You can usually strain the oil pretty easily with a fine mesh strainer with a coffee filter or paper towels. Do this twice if you need to. Air will oxidize oil faster than most people realize. I usually use a funnel and store the oil in the empty oil container and keep the lid on it.

You can reuse this oil if it does not smell rancid, you didn't cook too much stuff in it in the first place, and you didn't use too much heat. A lot of smoke, at a relatively low tempurature means the oil is not useable anymore. The more you use an oil, the lower the smoke point. You can raise the smoke point a bit by adding more fresh oil.

Just refrigerating oil alone whether used or fresh won't harm it. Some oils may become cloudy, but that does not mean it is bad, just that it is coagulating. I actually think it will keep longer because it will slow down the oxidization and breakdown from the natural acids in oil, especially pomace olive oil.

Pomace or pure olive oil is extracted using either heat, pressure and/or chemicals to get that last bit of oil out of the olives then filtered. This is a lower grade oil than extra virgin with a higher acid content which will break the oil down much faster, and can result in a bitter taste if over used. However this oil can be used quite successfully for deep frying, just that it won't last long and the smoke point is much lower than other cooking oils.

When cooking oil smokes, it is an indication that it is breaking down, and volitilizing. Extra virgin and other semi-refined oils smoke a lot (butter does too, unless clarified) when heated too hot.

Extra virgin olive oil really should not be used for general purpose deep frying because it has a low smoke point and will break down very fast.
Peanut oil and grapeseed oil have the highest smoke points, at about 400 farenheit, and are very good for deep frying. Canola, corn and cottonseed oil is good for general purpose frying but remember the smoke point for these oils ranges from 340-370 farenheit.

Good luck.

I use it several times before trashing it. If you cross the 400 degree mark or get it good and smoking, it's pretty much shot. Otherwise, there's nothing wrong with reusing the oil. I filter it through a fine mesh strainer into an old, clean bottle. As fatbuddy sez, you might not want to reuse oil that's seen fish, but for general purpose frying -- fries, chicken, tempura, etc. -- you can get three or four uses out of it. Once it darkens considerably, dump it in an old milk container or can and toss it.

Once of the more recent kitchen gadgets I got was a Fryer's Friend. It's basically a lidded container with a built in strainer on top. It's specifically for straining and storing re-usable frying oil. From Japan where they do a lot of frying.

It looks like this:
http://www.amazon.com/Endurance-Fryers-Friend/dp/B000F7DT2E

That is the best use for used fryer oil. Turn it into to BIO DIESEL

I use peanut oil for most of my frying, and when it cools down, I filter it through a strainer lined with cheesecloth. I fry a turkey every Thanksgiving, and I use the strained oil for all my frying needs over the next several months. It works for me.

Add me to the reuse it a couple times. Once you're done, if you have already thrown away the plastic jugs the oil came in, you could try using empty milk cartons.

i use a maxwell coffee can, and can the oil to the trash after the first use. problem is it's been so hot that fly's love the smell and I swear that my huge outside garbage can will fly away soon.