The trick to calming agents is to give them about a half hour before the stressful situation arises. If you wait until the dog is stressed, their own adrenaline will kick in and counteract the sedative.
Most de-stressers are very safe and mild and your vet can help you give the correct dose so your dog will be calm but not knocked out. Just like people, animals will each handle medicine differently. So you can always start with a smaller dose and see how that works.
There are also some natural remedies out there that some say work very well. Victoria Stilwell has a dog forum that is a wealth of information.
http://positively.com/forum/
Several members there use natural calming remedies and swear by them. :-D