You can't. That's why I'm always amazed by the vitriol unleashed when I tell people to go to top schools, get a HUGE scholarship elsewhere, or bust.
I think the problem with this advice is that you continuously give it to people who have other, non-Biglaw plans for the future (and have figured out a way to pay for these plans). I agree with you in that I have very little sympathy for people who go to a lower-ranked LS, taking out enormous loans in the process, and believe they will get a high-paying job when they graduate based only on the notion that "lawyers make a lot of money." Obviously such expectations reflect a failure to do basic research prior to beginning LS.
However, your one-size-fits-all approach doesn't apply to every poster here. Students committed to working in public interest, for instance, might have gone to the school with the best LRAP program, and may well have reasonable expectations about what they will live on once their loan payments are made. Others are fortunate to have family money (don't we all wish!). Non-trads often pay out-of-pocket, and therefore don't incur debt the way younger students do.
As long as students carefully plan to deal with their post-grad financial situation, why give them such a rough time?