I would say there are four qualities or abilities that will underpin a high score in a grammar school selection test. Conveniently they begin with the same letter - or at least with the same hard 'c' sound: curiosity, quickness, conversion-readiness, and carefulness.
Curiosity is observable in the extent to which a child enjoys reading - stories, histories, discoveries - and tackling puzzles, crosswords, word searches, maths puzzles, puzzles of any kind. Children who are actively and conscientiously curious prepare themselves for the Test without realizing it. They practise the thinking skills they will need, and they acquire the breadth of vocabulary that will be necessary for assured answering.
Quickness of thinking is required in all the Tests, if children are going to complete them without resorting to blind guessing. Quickness in arithmetic is essential to success in the Maths test. If a child has to work out multiplications and divisions (up to 144, or 12 x 12) she or he simply will not finish a sufficient number of questions in the time (only 30 minutes). Quickness of perception is necessary for successful non-verbal or spacial reasoning - and luckily, is more-or-less automatically picked up with repeated exercise! Quick efficient reading is needed for the English comprehension, and a quick eye for patterns in spelling and codes for the verbal reasoning.
Conversion-readiness is a skill that is perhaps less obvious than the previous two. So what do I mean by this? In Maths, for example, it is the readiness to see and to relish the fact that many things are the same, but just take a different form, or use different terms. A half is 1/2 is 0.5 is 50% is 'something divided by 2'. It is all the same thing. A pound is 100 pence. A metre is 100 cm, a 1000 mm, a tenth of a kilometre. And so on.
But it is not only in Maths that the readiness to convert is an advantage. In English and verbal reasoning it applies too, where conversion takes the guise of recognizing synonyms and equivalent meanings. Dark colours are sombre tones. Fighting strongly is resisting vigorously. To be able to express similar things in different ways is a higher order language skill not expected of 10-year-olds, but the ability to recognize how a word or phrase 'converts' or is equivalent to another, is an integral part of the grammar school test.
The fourth quality is care or carefulness, and this one may seem to go without saying, but it is worth dwelling upon and expanding with the help of a few more 'c's' - checking and correcting and being self-critical. In the feverish haste of the Kent Test examination, there will be almost no time for deliberate and painstaking checking of work - so the practices need to be internal and habitual rather than specially turned on for the occasion. Reading attentively so as not to answer inappropriately, checking that the sign in a calculation is a ' - ' and not a ' + ', and getting used to looking for correctable errors, misspellings or mistakes of grammar and punctuation - such are instances of the care or carefulness tested in the grammar school selection.
Few children at the age of 9 or 10 are naturally adept in all four qualities and skills: curiosity, quickness in working, carefulness in working, and skill in converting. All are capable of improving in them; some will always find the going hard. Grammar schools select those who, on the day, emerge most proficient in them on the evidence of the Test.