I am a thin girl with a breast size of 34dd and they hang. Is there a excersize or something I can do to make them more perky and firm?

No. The only way to lift them would be with a breast lift surgery. There are no exercises available.
Sadly no. Sounds like you are a good candidate for a breast reduction surgery. Unfortunately short of surgery or a good push up bra there is nothing else you can do.
A great bra. Invest in your breasts. Short of surgical intervention- you may consider going to a specialty bra shop. It is amazing what wearing the correct bra can do to support you and help you to feel more comfortable.
Shrink but not lift. Breast feeding can result in the breasts shrinking somewhat over time but they will sag more.
Breast lift. There is nothing short of surgery to help with sagging breasts. This can be done as a lift or a lift with implants.
Yes. Sashay to your favorite asps plastic surgeons office and exercise your right to feel good about yourself by having a mastopexy. Better yet, learn to look at yourself differently. At 23 you probably look great as you are. Do the operation, however if you develop neck, upper back, shoulder pain or as soon you start gaining weight.
Sorry, no. Unfortunately the answer is no. The breasts are just glandular and fatty tissue, so no amount of exercise will tone them up. There are lifts and other procedures that can help though.
No. Sounds like you need a breast reduction. Currently, nothing else will do. This is a very godd, very high satisfaction procedure. I have never had a patient regret it.
You can consult with. a plastic surgeon for a breasr lift or suspension. Increasing your pectoral ( chest )muscles with exeecises will also be quite helpful.,
Not consistent. There is no consistent or reliable method to achieve a perky breast without surgery or use of an external support garment.
Breast excercises. Sorry, there aren't any excercises that can change breast shape. The reason for this is that there is no muscle in your breast. You sould see a board certified plastic surgeon for a consultatin about a breast reduction. http://ocps.com/procedures/breast-reduction.
Breast size. Amanda23 without a posted photo or in person examination very hard to advise. Most likely no non surgical therapy can "lift" your breasts. Seek a visit with a boarded plastic surgeon in your area of texas.