Your adult child is an irresponsible brat; such is the nightmare of many parents. What can one do to prevent this? The authors of the Bill-Gates-recommended Parenting With Love And Logic have some suggestions.
The key is to make things the kids' problem from an early age. By taking on too much responsibility, parents let their kids off the hook from some key experiences at a time when failing carries little penalty. For example, kids will blow their allowance on the first day on candy and other junk, but they learn from these experiences. The kid that doesn't get this experience gets his car repossessed as an adult, so say the authors.
The first half of the book describes the authors' theories, while the second half applies these theories to common real-life situations where these can be applied. There is little in the way of data supporting the authors' views, it's all anecdotal.
But for the most part, I think the book makes sense. However, I think it has a fatal flaw when it comes to some types of problems outside the home. The philosophy lets the kid deal with real-world consequences rather than consequences imposed by the parent. But kids don't think long-term enough to get adequate feedback for their actions. For example, a child may be fine coasting through school with C's and D's because he can't see far enough into the future to know it's a bad idea. I think parents may need to impose artificial consequences in some cases, which goes against the book's philosophy.
If you have a child that's old enough to bargain with, you may benefit from this book.