First, you need to let your kids know that they are and always will be first in your life. In both word and deed. I cannot tell you how many kids I have seen whose lives have become train wrecks, as both parents pursue their own lives above all, heedless of the emotional needs of their children.
Second, depending on the age of your children, group recreational activities together can be a good way to break the ice and give your kids a positive memory with the new boyfriend. It goes without saying that whether or not he is good with your kids should factor into your relationship decisions.