I think I have just started conversations every now and again, and if the child responded then I would carry on, if not, I'd leave it for another day.
Dd1 had lots of input at school, so all I had to do was check she'd booked the open days she wanted to go to, especially if she wanted transport or company. I also got her a (cheap!) subscription to a journal about her subject so she had something relevant to write in her PS 
Dd2 was (is) very organised and motivated - but I probably talked to her the most about the whole process as she liked to discuss everything in great detail. I involuntarily became quite the expert on the structure of her chosen course at several universities! Her school were less proactive, and I was more supportive/'pushy' with her, suggesting things I'd mostly read about here, such as masterclasses, UNIQ, Sutton trust summer schools etc.
With ds, he's obviously seen the process twice now and has absorbed it a bit, and the girls have talked to him about it all, so it's not come out of the blue. But I've tried to tread quite lightly with him over y12 as he's been up and down in mood over the last couple of years, and I am quite nervous of feeling like I'm putting pressure on. But he's less good at getting things started than his sisters, says his friends don't talk about this sort of thing etc, so I have now got him to sit down and look at open days with me, and been quite pushy, "right, we're booking this one".
So I guess it's all about responding to whatever your dd seems to need from you 