We found co-sleeping was the only way to ensure any sleep when our son was small.... even now (he's nearly four) we often have one parent sleeping with him, as this guarantees better sleep for everyone. It often seems odd to me that adults are expected by society to share their bed with another person (married couples who admit to sleeping in separate beds get some very negative responses!) but conversely children are expected to sleep alone. In my experience, a small person needs the loving presence of another person to sleep well more than a big one does!
In addition to this, I find a good bedtime routine essential and being as firm as possible with sticking to bedtime. If a child gets overtired, then falling asleep becomes next to impossible!