Don’t fall into the prestige trap (e.g. child MUST attend the highest-ranked university they get into; child MUST apply to the most competitive and highly ranked colleges, etc.).
Relatedly, help your child find the right fit for them. He or she will take advantage of more opportunities, and will be more successful in a place where they fit in, versus a place that looks great on a resume but is not really a place that motivates the child (4 years is a long time to find motivation to work hard in a place you do not like, that one feels does not like them, etc.).
You know your child, you know their weaknesses. That is where you can help, by addressing those, putting your brain and time into those things (e.g. so many deadlines, so much data that has to get out the door, a parent can really help organize those commitments). Interfering in other areas has the potential to backfire and increase stress.
There is no guarantee that anyone will get into a competitive school (think admission rate under 20%, and definitely those under 10%). I don’t mean one’s favorite, or a handful. I mean just one. It’s hard to do, and it appears that every year it gets a little bit harder.
Be ready when feelings get hurt or dreams (as in dream school) get crushed. They will need you, if only to know that you still support them.
Putting things under greater pressure can lead to explosions. If your child is high-achieving, don’t add to their pressure or their stress. Try to be a release for it. I have seen and known kids who simply buckle late in their junior or senior year under the pressure. In those cases, parents often sometimes see how they added to the pressure.
Don’t live through your child. This is their process; it’s about their future. It’s not about you.
A more practical one: don’t apply early just to do so, or because you or your child think it’s “easier” to get in. It’s more complicated than that. So only apply early if your child truly loves the school and it’s the #1 choice that will not change. In my opinion, this applies even if the early application is not binding. One caveat is that sometimes state schools can be different. But don’t assume so.
Finally, forgive the mistakes. You will make them. Your child will make them. It’s okay.