When we study Romans 8:29-30, we usually agree that it is not a claim of determination of who will be and who will not be conformed to Christ. It is a determination of what will happen to those who are called (by their choice, Romans 8:5-11). They will be conformed to the image of Jesus Christ and be glorified. God predestines the consequences of the choice to be in Christ or not; but he does not predestine the choice itself.
An easy-to-remember rule on this passage is "It's the plan and not the man." That rule promotes the correct understanding on Romans 8:29-30. It still may be a little difficult to wrap the mind around.
In a recent Facebook conversation, a participant (Adrian Murzac) pointed out the difference between a type and a token. A type defines something that applies to a group of objects. A token is a particular example within the group. Understanding the difference, I think, really helps us appreciate this Bible passage. Consider a flock of birds as it flies around in the sky like a cloud--almost like a single organism. They fly together corporately. The flock represents a type of bird. Now, focus on a single bird in the flock. That's a token. Consider a school of fish in the ocean that you may have seen on a nature T.V. show. When a shark swims through the school, the school of fish parts and makes a kind of tunnel for the shark to swim through and the shark is unable to catch a single fish in the school. The school of fish is a type while a particular fish in the school is a token.
What God predestined in Romans 8:29-30 is a type of person, not a token (particular) person.
God predestined a people conformed to the image of Jesus (a type of people). He did not predestine any particular individual's choices. Paul never said anywhere that a life choice with eternal-consequences has been pre-made for anybody. That particular choice must be made by the individual person (token). Have you made that most important choice or are you waiting for something?