HP, I think 'You tried so hard' is very generic and has a ring of inauthenticity to it (if that's a word). If I were to discuss a test result, (I wouldn't because I think the preparatory effort itself is the main thing and everyone has a bad day occasionally even with solid preparation) I would only ask the child how they felt about the result. Two or more Fs even with great prep and I would be wondering what was missing from the approach, because patterns are important. I agree praise for effort can come across as forced or artificial. Maybe the trick is to not comment about every single squiggle your child produces. Too many comments makes a child think everything needs assessment and feedback becomes the object, which defeats the point just as much as praise for results.
Up to about age 12 or so I never mentioned their clothes (I bought them all after all) or how they wore their hair apart from basic decisions about hair style, or made any other remarks about their appearance unless we were going somewhere that demanded a certain style of clothing. It was the done thing that everyone took a daily shower and brushed teeth twice a day and after that they could do what they wanted. I think there is a time when children appreciate and actually can use a little feedback on their appearance from someone whose judgement they trust. Up to puberty, I don't think appearance warrants comment. Handsome is as handsome does.
When my children got to be teens (youngest is now 14) they had indeed already absorbed many messages about what 'beautiful' is. I made a point over the years of never commenting on appearance and ignoring celebrity culture, but it seeped into their lives to some extent anyway.
Someone gave us a copy of Teenage Beauty by Bobbi Brown when DD1 was about 12 and it was a really great, sensible advice, basically about bringing out your best features. I liked it because it assumed everyone has at least one feature they can show off or feel happy about. I acknowledge that how you feel about your looks is important, but I do not think it is wise to imply that there is any definition of beauty; if you are happy about how you look, that's all that is important, and that can arise from specific advice and approval of specific elements just as much as blanket 'you're beautiful' statements. Maybe even moreso. There comes a point in every young teen's life when 'you're beautiful' just sounds like mum being kind. I am sure even models occasionally look at themselves in the mirror and find something to be bothered about.
My DDs liked long hair, nail polish, makeup it made them happy and I was happy to see them happy. If it flattered them (or otherwise) I said so, and tried to be detailed and specific 'that eyeshadow brings out the green in your eyes' or 'I think that lipstick swamps your other features' or 'I think dark blue washes you out'. I have two redheads and one strawberry blonde among the DCs, and was able to point them in the direction of mascara and eyeshadow that solved problems they felt they had due to fair eyelashes. It definitely gives confidence when you have dealt with some problem or other. I would not have helped them if I had told them they were beautiful and left it at that, and any other pronouncements I made on appearance would have been discounted.
I am not a mother who objects to leg or armpit shaving or experimenting with different looks, whether they do it or whether they don't. They know I have their back when it comes to putting their own version of their best foot forward, appearance-wise. I will traipse around twenty two shops to find a prom dress that makes DD2 feel she has nailed it. To me it's a question of supporting them as they branch out into the world, not narrow focus on achieving a certain look.
They also know I also appreciate many other elements about their approach to their appearance and personal maintenance -- that they take good care of their teeth, that they wear sunscreen and don't try to tan, that they spend their own money on all their makeup (earned from babysitting and other part time jobs), that they research products and get good value for money.
I think there is an element of saying 'you are beautiful/clever, etc' that is really harmful, and that is the implication that there is a specific standard of 'beautiful' or 'clever' that can be aspired to, or in the case of a 7 yo child hearing it, believe they have attained. I think it is important not to give the impression it's something fixed in stone. 'Beauty' whatever it may be is in the eye of the beholder, and what may be popularly considered 'beautiful' can be fleeting. Wrt 'clever' I think it is really important to avoid giving the impression to a child that there is such a thing and they have attained that status. Double edged sword is a phrase used by a pp^^ and I think that sums it up well.