I've got two boys aged 4 1/2 and 3 that share a room. We have had the same problems. We have developed a consistent nightly routine. We give them a bath (I think the warm water starts to help them wind down), then get them into their nightclothes, brush their teeth, read scriptures with them (5 versus a night), say prayers, and then do a family chant where we all put our hands in and yell our own little motto "to eternity and beyond!" as we throw our hands in the air. Then it's hugs and kisses and into bed. They each get to choose one toy and one book to take to bed with them. I think that that is very helpful as well because it gives them something to focus on which calms them down. They love their nighttime routine and will remind us if we miss anything. I think that kids really rely on routine to feel as if they have some control over their lives.
We usually give them about 20 minutes to fall asleep but if they go longer or are very loud, we do separate them. We just bring one of them into another bed until they are asleep and then move them back. Since they know they will be separated if they don't behave, they are much better behaved.
Nap time is a breeze because they are so used to that routine as well. And if they have a problem with it I just remind them that this is what we do every day and today is no different. Or I'll point to my watch and say, "see? it's time for resting". That works at night too and helps them to accept bedtime because they think, oh, it's not just because mommy said so but because that's what time it is.
Well, it's a little early in the am for me so I babbled a lot. Hope you find something helpful in here.
M.