How to reduce stress and boost immunity

My two dogs are going into boarding kennels for the first time in a couple of weeks. Is there anything I can do to make it less traumatic for them? I've heard of a collar they can wear that gives off reassuring aromas but can't remember what it is called!
I've also heard that while the dogs have been vaccinated against kennel cough there are other strains that they won't be protected against. Is there any supplement I can give the dogs to just generally boost their immunity at what must be a very stressful time. Anything homoeopathic I can give them to get them through this and make me feel a but less guilty about not taking them on holiday with me!
Glenda Kennedy, Inverness