Remember the more you know what to expect the less stress you will endure, so bare with me.
As soon as you're notified that your dress has arrived at the store you should make arrangements to try the dress on, at the store. Always have a bra on even if it's your everyday bra.
It's not often that a dress fits perfect. Straps and halter adjustments as well as a hem are almost always a given. Our bodies are not perfect according to the manufacturer size charts so a tuck here and a let out there are very common.
Length of dress - Rule of thumb for full length dresses is that the dress be just touching the floor to 1 inch off the floor. T-length dresses typically don't require a hem. A common question regarding a T-length dress is, "Does it matter that some girls are taller than others?" It's a personal preference. I've seen wedding photos of bridesmaids with T-length dresses not hemmed to a specific length (i.e. all 12 inches off the floor) and everyone looked great.
Straps or no straps - Even if you choose for the most part not to have straps on your dress I think it's a good idea to have them sewn in for the reception. Tuck them in so if and when you choose to wear them they're ready to go. Like any garment, from body heat and movement, your dress is bound to get somewhat roomy by the end of the day.
Undergarments - It's most important that you provide support for the "girls". If they aren't doing their job however assisted, you'll have problems. I recommend a good strapless push-up bra that fits. It has to be tight in order to do what it's supposed to do. Cups can be put in most dresses which I do not recommend for anything above a "C" cup. If you feel you need to smooth things out in the midriff area get something made of lycra (swim suit type fabric). It should extend over your hips or it may roll. Stay away from the padded full length bustier, they're uncomfortable and just add bulk.
Bustle - What is a bustle? A bustle is put in a bridal gown to pin up the train and allow you to move freely after the ceremony. This usually consists of 3-7 hooks and loops or ribbon ties. The standard bustle is one that is put up on the outside of the dress placing all the excess fabric on the caboose. The french bustle or under bustle is a much more functional bustle, much easier to put up and less apt to come undone. I always tell the brides there's no guarantee on the bustle. We basically put a bustle in so you know where to pin it after someone steps on it and rips it out.
Crinoline - A crinoline or can-can (tulle half slip) is used to poof out the bottom of the dress to show off the detail in a bridal gown. It has also been known to be used as a tool for shortening the dress. Not every dress needs a crinoline and not every bride wants to wear one. I recommend using the hoop. It remains consistent morning to night and doesn't stick to your legs; in fact it's rather breezy. A can-can tends to lose it's consistency by the end of the day due to body heat and humidity which in turn will lengthen your dress.
Shoes - Go for comfort! Remember you're going to be in them a whole day and if you change into slippers for the dance your dress will probably be too long. Wear the shoes for long periods of time prior to your wedding day. Break them in with damp socks while doing dishes, vacuuming or any other not too physical house chores.
Common Scenarios
Your dress is a bit snug and you just can't quite get it zipped all the way - "Get the girls up" while zipping, it makes a world of difference. Take the bra off once it's zipped it might be adding more than you need. If this is the case and you feel comfortable not wearing a bra you will need to get dressed with the bra and take it off once it's zipped the day of the wedding.
Your dress is a bit too big in the bust - A push up strapless works wonders. Cups work well for someone who needs just a little extra in the "girls" department. I've found that some of my customers need both the bra and the cups. Last but not least, add straps.
Your dress fits great in the bust but snug in the rib cage, waist and/or hip area - Most dresses have plenty of extra side seam allowance and can be let out. As long as you were able to zip the dress you usually only need ½ to 1 inch let out for comfort.
You're going to have a baby and you may or may not know this when ordering your dress - I recommend ordering no more than one size bigger than your bust at the time of ordering your dress. Order extra fabric and lining to be used to re-cut complete panels rather than putting in a gusset (an inserted piece of fabric). You may need to empire the bodice depending on the style of dress. If you order a dress to fit your belly you'll have major problems with the bodice and the dress still might not be big enough to go around the midriff area. Always check with your seamstress to see what they prefer.
Worst Case Scenarios
Your dress has arrived and it's ridiculously too big or too small, what do you do? First and foremost don't panic, you will have a dress for the wedding. Second and just as important whenever possible try the dress on at the store you purchased it from. If you took the dress home you will need to contact the store you ordered your dress from and make arrangements to have someone who knows what they are doing help you, preferably the owner and/or an in-house seamstress. Without physically seeing the dress on you, no one can properly assess the situation.
You'll need to be re-measured at the store you purchased your dress from to see if there's been a weight loss or gain. If your measurements are the same you should try on the sample that you tried on when ordering the dress. If the sample still fits it's safe to say that the dress you received was miss-sized at the manufacturers. This does not happen often but it does happen. Time permitting a new dress should be ordered for you and you'll be back on track. If there's not enough time to re-order, the store should correct the error by having the dress altered to fit.
So if you lost weight congrats and if you gained you're still beautiful. Regardless, if there's a will there's always a way. In all the years I've been in the world of weddings I have yet to have a wedding without dresses.
Through out the whole process of planning your wedding, remember we are all human and unless nothing is done to correct the situation it's best to trust that things will work out. Trust me, things always work out!
Check with your seamstress to see when you should start alterations. Typically it's 6-8 weeks for bridal and 4-6 weeks for all other. Again check with your seamstress and at least let them know you may be in need of their services.