Okay, so you got the interview! Now let's talk about how to best prepare yourself to impress everyone you meet and get that offer:
General advice as you start to prepare
Pace yourself - don't interview at more than 2-3 companies at once
Once you make it to final rounds you'll have full-day onsites
I recommend spending at least several hours preparing for each onsite
Give yourself time to practice
Despite how much time I spend on this stuff, I still don't perform well on interviews where I don't take some time beforehand to practice / rehearse stories
Start with a couple companies you wouldn't be upset about not getting offers at
Good practice for the interviews you care most about
While I do believe services like this one are highly valuable, I actually recommend only doing it if you've taken plenty advantage of free resources first!
Read this guide, watch some prep videos on YouTube, do an interview with a company you don't want to work at as practice, etc.
Only after that if there are specific areas you still feel you need work on, services like this could help you go the extra mile