You need a guy that loves the game, the number one reason is that the kid got paid and he's had enough. If they manage it right they're set for life. You need a guy that loves waking up at 5am, hitting the gym and spending a good part of the day watching film on their tablet. If you can find a guy who has that hunger to get better every snap you're already halfway there.
Drafting based on measurables is the other downfall(Maybin comes to mind, bulked up for the combine when everyone knew he couldn't keep the weight on) I still to this day will never understand how a guy running a 40 in his underwear propels him up draft boards. Put on the tape, if he looks the part, and has the build, take him.