One thing worth doing is adjust the sleeping environment. Reduce the number of soft toys to just their favourite, but make sure that one's in bed every night.
Drop the light and noise level by getting proper blackout curtains up, or get a low glow nightbulb or some ambient noise going on. Find what level of noise your child prefers.
But the key thing is consistency. They need to see their bed as their space. It is more worthwhile YOU SLEEPING NEXT TO THEIR BED IN THEIR ROOM than them sleeping in your bed, unless you have the capacity to sleep through them, because at least they're getting comfortable with the bed. You don't want that to be a long term thing, but it's at least giving you an out that you can sneak out once they're asleep.
Another thing to try is to spend some time during the day reading to them while they're in their bed, so they start thinking of their room as a nice place.
CreationEdge's answer is a brilliant summary, but the most important thing is that you have to be comfortable with your plan. If you're cracking and allowing them back into the bed, it's a sign not that the plan won't work, but that you can't commit to it, because it's not right for you.