If you need time to think then ask for it.
Don’t make a rash decision, give the hiring manager a time frame and stick to it. Speak with your recruiter, your peers, people at home and circle back in person; no matter what the decision is.
Realistic negotiations are often part of the process.
Voice your questions to your recruiter; we are well placed to manage this with you. Often we already know if there is any room for movement on salary, vacation days etc. But be realistic; refer back to your target job requirements and why you wanted a new job in the first place.
Get everything in writing.
Make sure you read your offer details thoroughly and confirm everything via email.
This is the next chapter in your career and what you have been working toward. Convey this to the hiring manager to further underline that they have made the right decision.