When purchasing a rifle scope, there are several main factors you should consider.  There are a lot of major and minor considerations when buying this equipment.  First, we'll take a look at the major items to consider.  Obviously, look at your budget.  Some scopes can be bought for less than $200, while some run well over $1000.  You want to be sure that you spend as much as you can, without breaking your budget.  Second, you want to look at the size of the lens that you want for your scope.  The larger the lens, the higher level of power you will get.  They also translate to larger viewing areas.  Next, look at the power and settings of the scope.  You want to be able to use your scope and quickly adjust it easily.  The more intuitive your adjustment options are, the better and faster you will be able to acquire your target.

There are tons of options available, so when you pick your scope, make sure you choose  one that helps you aim, acquire your target, and can stand the test of time.  The best rifle scopes are light and rugged, with great lenses and optical quality.  The image quality is absolutely vital, as this is the reason you're buying the scope.  The more light that enteres the eyepiece, the shaper your resolution will be and more accurate your shot will be.  Lens coatings reduce reflection, which causes glare.  You should buy a lens that has at least a single coating on it.  

Furthermore, check to make sure your scope is fogproof and waterproof.  This will save you heartache and repair costs in the future, not to mention higher quality shots.  Finally, make sure that the scope warranty gives you solid coverage.  Manufacturers often give lifetime warranties on their products.  There is a lot of competition out there, so this is a major consideration as well.  If a company is willing to replace your product if it fails, then that can save you hundreds of dollars right there.  The bottom line here is to use common sense.  As long as you stick with the brands with good reputations, you will probably be happy with your new scope and the shooting experience it gives you.