Mine are very, very bright, it was a bit of a shock when the teachers started raving about the eldest as DP and I are extremely average. We don't do any academic pressure or tutoring, eldest was much lazier but still got all A* and 1 A at GCSE. We did talk to them a lot as children, DP was very good at answering questions properly, you'd be surprised how easily they just absorb stuff. And lots of books.
The stark difference in work ethic between them proves in my mind that some kids just learn more easily. She would just pick up a language by reading street signs and listening to foreign radio, then reading books and "thinking" in it. It's a bit surreal, but I'm more used to it now.
We really are incredibly lax on academic pressure as both work full time and trust them to just get on with it. Luckily they were both bright enough not to give us anything to worry about, so I would emphasise talking, answering questions and reading books as young as possible. My DM taught her to read before she started school and I always wonder if that could be the reason.
Also- SLEEP! It's so, so important for them to get as much sleep as possible through the primary years, it was one of the things we were strict on, as is so vital for brain development. In bed by 6:30 every night until they were at secondary. There were a few studies on this that pushed us to be firm about it, but obviously my sample size is only 2. I don't know if it made them clever or not, but it is very important for their development.