Definitely agree with previous posters -
But the important thing is, to be chatting regularly, and being interested in "their stuff" even if it's not really interesting and getting used to letting them tell you what they think of something, rather than jumping in with your opinion
^^ This especially.
Something else I do with my DD11 and DD13 is a Bedtime Book. I started it because dd2 right from an early age would often leave me notes on my bed after she'd gone to bed, about anything that was bothering her, questions she'd thought of etc.
It got me thinking that the time we do most of our thinking/worrying/processing is as we're going to sleep.
So, each of my DDs has a special, hardback notebook, sort of like a diary, and if there's ever anything they want to tell me/ask me/ complain about, they can write it in the book and leave it on my pillow at night. I will then write a response and put it back on their pillow in the morning.
We've been doing this for a couple of years now and I love it, it really works for us.
It by no means replaces actual conversations, but I think they find it much easier to bring up difficult/embarassing things in this way.
We've had worries about bullying, periods, poorly grandparents, homework, friends - to stresses about outfit choices for parties and what to spend birthday money on. Sometimes even just a little joke or a picture.
They kind of treat it like a diary I guess, but one where they get an answer.
So far there has been nothing too shocking or dramatic but my hope is that if anything serious did ever happen, this would give them the means to tell me about it.
It has also had huge benefit in improving their writing!
It might not be for everyone - not sure how boys would react? But I think it's worth a try 