A huge trend in the past year is in re-styling or altering vintage frocks for weddings.
It's very important to be sympathetic to the original garment no matter what you intend to do with it. Treating it with care and respect will pay off in the end result. All this seems pretty obvious...but you'd be suprised how often this doesn't happen.
Hand washing it in mild soap by hand will give you the best results unless the garment is dry clean only - in which case send it to a reputable cleaner. If you do not wash the garment and then go about matching new fabrics, thread and trimmings you might be unpleasantly suprised when you eventually do clean it as your cleverly chosen colours might not match it.
It's also really important to be really careful when unpicking stiches, as old fabrics will be more likely to rip and tear if you man handle them. Since there is no way in hell you will be able to match the fabric it's best to be over-cautious.
When you adapt vintage to a modern figure it's always useful to remember that a lot of the vintage shapes are created primarily with the correct undergarments. Really satisfying results are acheived when the essence of these underpinnings are copied and the garment fitted over the top. An hourglass vintage 50's dress for example is not worth taking in unless you can re-create the shape it is supposed to make with corsetry and petticoats and the twenties flapper look completely unacheivable without flattening the chest. Unless of course you are very lucky and god gave you the correct bodyshape for the look that you love the most.
Taking note of the way it is put together, and remaining as true to that as you can can be a very good learning experience. I love looking at old clothes - I find it such an education, and always inspiraional. Try to replace anything with materials appropriate. For example repairing an old corset with rigelene would be a total waste of time.
I also think it's really important to remember that antique clothing is just that - antique. It's not a throwaway item like somethng you would get from today's high street. It has survived the ravage of wear and tear and time, so treat it with the respect it deserves.