I think the hardest part is figuring out what YOUR body responds/doesn't respond to. After that, just be consistent.....
Good point gordiano... but if you've done enough study (sometimes in fields outside of nutrition) you can actually quantify a lot of the "variables" involved... there isn't enough genetic variation among modern humans to produce significant variations in the basic biochemistry involved. What confuses people are the small racial variations and the HUGE variation in dietary habits; hormone profiles; and the resultant effects on training response.
When I work with a precontest (or offseason) bodybuilder I do a full evaluation before I ever give any nutritional advice.
Body structure; joint measurements; tendon attachments; joint flexibility; limb ratios; bodyfat measurements; relevant medical history... anything and everything that might be important.
Then I evaluate all the weird little anthropometric nuances that only geneticists are interested in... finger length ratios, jaw structure, foot shape, calf structure... they all hint at the underlying genotype, whether it has been fully expressed or not. It's a simple matter of diagnostics.
After a full physical evaluation, that's when I advise a basic healthy diet... and then I evaluate the trainer's workout regimen. Specialised dietary recommendations are my third priority, because so many people make huge progress once the basic mistakes are removed from their bodybuilding efforts and I don't want them to think that the ten pounds they gained almost overnight are due to my recommending vitamin C when in fact it's down to them finally squatting and deadlifting properly. I prefer to convince people that I understand the proper implementation of the basics rather than attempting to build myself up as some kind of enigmatic oracle.
If more coaches and trainers simply kept up with the cutting edge science we'd have a lot more healthy, well developed natural bodybuilders who never had to resort to steroid abuse.
The Luke