When the bedtime routine is over, do not allow your child to get out of bed or leave the room unless she needs to use the toilet. And do not rush back in to rearrange the covers. You've already offered one last chance for verbal requests; now's the time to let your child find her own ways of relaxing -- playing quietly in bed, listening to music, looking at a book, and so forth. If she starts crying or calling for you -- and you know she's in no real danger -- tell her you need to shut her door until the screaming stops. If it continues for more than 15 minutes, reopen the door to reassure her, but calmly repeat your intention to keep the door closed until she settles down.
If your child tries to leave his bedroom or crawl into your bed, take him back to his own bed immediately, with as little fanfare and fuss as possible. Unless he's extremely fearful or sick, remind him of the stay-in-bed rule and warn him that if he gets out of bed again, his door will be shut until he is back in bed and quiet. Never threaten or yell at your child, however -- it will only ignite his ire. He needs to know that you understand how he's feeling and are trying to help. You might say, "I know it's hard to stop playing and go to bed, but we all need to rest. Mommy and Daddy are going to go to sleep soon, too."
The key is gentle firmness: You needn't raise your voice, but you must stick to your guns -- even if, at first, that means carrying your child back to bed every 10 minutes like clockwork. Most kids will give up the fight within two weeks if you remain firm and don't bend in the face of their fears.