Wow~This could have been me writing this a few months ago. I have a 3 1/2 year old and a one year old as well. My older boy has had much more trouble getting to sleep and staying asleep than my little one, but my husband found the answer. I'm sure it's in one of the sleep answer books out there, too, but here's what worked for us:
It got to the point that we (one of us, not both!) had to actually lay down in his bed until he was fully asleep then try not to make any noise getting out of his bed and room, then when he rolled over later and found one of us missing, he'd wake up and call out for us, or run into our room screaming! Clearly this was unhealthy for us all.
My husband and I talked to him first and explained the new plan and why things had to change: everyone needs proper sleep and when we don't get it, we're all cranky. Nobody wants to be cranky, right? Right! So from now on, I'll get farther away until you can go to sleep on your own, ok, big boy? OK.
The first few nights, he sat on the edge of the bed. Then for a few nights, he sat on a chair in the room. Then for a few nights, he moved the chair outside the door with the door wide open. Now, we say goodnight and go do our own thing... go straight to sleep ourselves... watch tv, read, whatever.
I am a huge advocate of a nighttime routine. We have a bath shortly before bed, then read a book or two (or three or four, depending on the time) then we get into bed and say prayers, and my older boy has always liked music, so we have a really nice cd with lullabys and the backround beat of a heart. We put the music on and say goodnight. That's that. Now, sometimes he still gets up or calls out, but for the most part, he's in bed all night and everyone's a little less cranky in the morning.
Hope this helps. Good luck.