Given that he does understand when we speak with him, I use problem-solving techniques (Dr. Thomas Gordon's P.E.T.) with him, but he still comes into our bed while we are sleeping.
Lately I have resorted to bribery, offering chocolate if he does the night in his bed, a ridiculously limited option given that it will not work over the long term.
You see, I know all this stuff, but somehow, I am having great difficulty applying it. I would welcome any suggestions you might have.
Richard Ferber's books deal specifically with the sleep problems you are dealing with. Clearly bribing him complicates the problem as you have stated. I would suggest a 20-30 minute wind down routine that involves quiet activities in his bedroom, like being read to and exchanging stories. Do anything you can to associate his bedroom and his bed with things that are comforting and exciting; making a big deal out of his "big boy bed" is a must.
When you do finish your wind down routine and it's time to go to sleep, you or your husband can tell him you will lie down(or sit) next to him while he falls asleep and then you'll be in the next room if he wakes up. If he does wake up, talk to him soothingly and take him back to bed where you will stay with him while he drifts off to sleep again. This may seem like it's too much of your time but this can be a scary transition and one that merits extra special attention, time and understanding. Good luck, I'm sure you will all come through this famously.