First, I do not recommend using a cover for outdoors because all, even the very best, eventually get dust and dirt on the underside and in the wind, even a custom fitted cover will flap around a little and the dirt will start to scratch away the paint.
If you must get a cover DO NOT get an off the shelf generic cover that will fit any average car. These covers may say the can be used outdoors but will destroy your paint from a lot of flapping around from the wind.
Get a good fitted cover from some major cover manufacturers.
Crafted2FitŪ - Covercraft Custom Fit Car Covers
Coverking - Custom Car Cover Materials
Outdoors a tent is best but might be a bit impractical depending on where you leave your car.
Car Covers and RV Covers
Finally here's a list of tips for storing cars over the winter, many more if you search for them.
http://www.rochestercorvetteclub.org...-checklist.pdf
It was made for vintage cars so some items aren't really applicable.