First, look at the dress. Does it have beading, seed pearls, sequins, rhinestones, or anything that might be heavy sewn to the dress? If it does, it must be stored flat so that the weight of the trim doesn't tear the material. The usual way to manage this is to wrap the dress in a white sheet, then store it under the bed. A long, flat box will protect the dress from cats and dust bunnies. You can fold the dress, but if it cooperates, rolling it will leave fewer wrinkles (this trick also works for packing clothes for your honeymoon).
If the dress has heavy fabric suspended relatively weak fabric, it is also a good candidate for the "under the bed" treatment. A heavy dress with narrow shoulder straps or a dress with net insets in the bodice are examples. You don't want the weight of the hanging dress to pull it apart.
If your dress is relatively plain, your next step is to check the fabric content. Natural fibers need to "breathe," which they cannot do in a synthetic garment bag. A dress of silk, linen, cotton, or wool should be wrapped in a cotton sheet. You can leave a plain, not-to-heavy dress on the hanger, but once you start running around with king-size sheets, you may find it easier to once again store the dress under the bed.
An acetate dress should also be kept away from heat, sunlight, and water. Although acetate is a synthetic fiber, it can easily be weakened. If "under the bed" would put your dress next to a heat source (floorboard radiator, electronics, electric blanket), choose another hiding place. Acetate will be happiest in about the same conditions as a good wine.
If, however, your dress is good old polyester, with little or no trim, it can stay happily on its hanger in a garment bag for seven months. A very heavy dress might start to pull apart after seven years of this treatment, but polyester is remarkably stubborn stuff. A 100% polyester dress with no appliqued trim can be washed in the washing machine and allowed to drip dry, too.
When faced with a dress with a mixed fiber content, follow the procedure for the most demanding fiber. This will usually have you pushing sheet-wrapped packages under the bed!
Pull the dress out from under the bed a few days before the wedding, and put it back on its hanger with the train draped over the hanger. Wrinkles will sometimes "hang out" by themselves, saving you a great deal of hassling with an elaborate and unwieldy garment. If that doesn't work, try hanging it in the bathroom with a good hot shower going full blast: steam and the weight of the fabric will usually do the trick. (Be careful that the dress itself doesn't get wet.) If all else fails, there are special steamers that you can rent to use on wedding dresses. A dress that wrinkles when you look sideways at it (linen will do this, as will some forms of silk) calls for a steamer that is brought to the ceremony site. Make sure that the layers of crinoline under the skirt are also fluffed out and unwrinkled.
Steaming will also do wonders for a veil that has become wrinkled.