allow your pup to take its own bed and toys with him, and as luba said put in something of yours...
i also find if you leave really really detailed instructions and advice how to handle pup it wil help your friend no end, also be very clear about feeding and snacks as you dont wnat underfeeding or over to occur...
and to help you i find the best thing to do is ring your carer once every day jsut to say hi and check on pup, make sure you tell them also to report all probs and not be afraid to discuss stuff (they may be afraid of offending you)..
i call my girl once a day if i leave her anywhere and i find it reassuring...
have a good trip
and dotn worry too much, its hard but you will get through it.