Saree is indeed one of the most elegant wear for women. Not only saree draping is an art but also its shopping needs a special care. In order to buy a perfect saree for a particular occasion or just like that, one should keep in mind the following important things.

Weather
If you are too picky about weather then follow these basic rules:

Pure silk are best for winters.
Cotton saree will help you feel fresh during summer.
Synthetic saree are good for rainy seasons for the simple reason they tend to dry fast.

Occasion
All women have a good sense of styling. Still saree can be categorized as per occasion too

Heavy embroidered and designer sarees are best suited for weddings, semi-formal occasions or celebrations.
Less embroidered can be chosen for parties and other celebrations.
For casual occasions sober sarees should be bought.

Designs & Patterns
There is however no hard and fast rule to select saree as per design and pattern. All sarees look good on every woman if worn properly. And one should always go for ones choice. Whatever one feels good in.

Georgette, chiffon and silk sarees can be opted to look slimmer.
Cotton, tissue and tussar fabrics can help thinner women to look a bit fuller.
Big borders can help taller women to conceal some inches.
While thin borders or no borders can add some inches to your height.

Color
The above rule applies here too. Always go with your heart. Truth is there is no limit to experimenting, if you have a favorite color or may be know your loved ones favorite then go for it. Still the following are the basic rules:

Dark skin women should go for dark colors.
For the fairer complexion you have the range from pastel to brighter shades.

Finishing up the Saree

The bottom edge of the saree needs an additional inside border stitched called fall. This should match the base color of the saree. It will keep your saree in shape once you have draped it and also prevent it from getting damaged.
If saree ends are unfinished they need to finely stitched known as pico or edging to give it a perfect finish.

Matching Petticoat and Blouse
Saree is worn with matching petticoat (inner skirt worn below the saree) and blouse (upper garment). The following things should be kept in mind about them

The petticoat should ideally be of the same base color as of the saree.
Rich net sarees mostly comes with matching shimmer petticoat fabric.
If the blouse is to be cut from saree it shouldnt affect saree width.
Their fitting is also very important. So if you are buying them online read the instructions of taking measurements to get them stitched carefully.
Also look for the customization of blouse and petticoat if you are buying saree online. Also dont go by just with the image of blouse shown there on site. Clarify whether you will get the exact design or not.

Innovative and Designer Sarees
The innovative saris (lehenga pattern, ready pleats etc.) differ only in pattern and design from the usual sari that we come across. If you are not sure about draping the sari then these are the saris for you. They are easy to drape and manage whilst looking gorgeous and elegant. Clarify your all queries e.g. how to wear them before you buy such sarees.

Online Buying
Now if you are buying a sari online for the first time consider the following points too:

As a buyer you should look for maximum information available about the sari (colour, combination, pattern, embroidery, length, material) or simply ask for them using live support, mail etc.
Look for enlarged images of the saree which gives an understanding of the material.
Consider the customization, shipping, postage and packing costs to determine the right price.
A secure way of paying should be there.
Check privacy statement.
Make sure you know the traders full address.
Read the refund and return policies.
You should be sure that the store has refunds or exchange option in case you didnt like the saree or any of its customizations after the delivery.

Check Your Saree Thoroughly Once Its Delivered to You
To avoid any kind of issue make sure you check the following things in your saree as soon as you get it delivered so that you can ask for alteration or refund. However most of them applies equally when you buy saree from a local store.

Always ensure yourself about the optimum length of a saree. Shorter sarees can not make enough pleats (min 5-7) which in turn do not grant the so much appreciated grace of a saree. The average length of a sari is 5.5 m, and its width is approximately 1.5 m. (Note: Some designer sarees like lehenga pattern use to be short as they dont require pleats)
If you are very tall, check that if its enough wider.
If you are going to get the blouse stitched by yourself and have bought a saree that contains extra cloth for making a blouse, always ensure to measure the sarees length after deducting at least one meter cloth from it.
When buying embroidered sarees, see there are no broken threads in between.
Check fall and pico are in place.
Ensure flitting of blouse and petticoat.