Bedtime routine! Develop a short but very consistent bedtime routine that you can use anywhere. When you use it regularly at home and condition your baby to go to sleep with that routine, you can then use it at a friend's house, on a trip, anywhere. Add in a "lovey" to have an extra bit of familiarity at bedtime.
Baby sleep. This can be difficult but the best way is to stick to the baby's routine as much as possible in the new environment. For example, keep the same sleep schedule, do an evening bath and reading time before bed, use a blanket or toy that the baby likes to sleep with, use a sound machine, keep the baby non-stimulated in the evening times, etc.