What is the best advice you would give to an individual getting ready to be promoted to Sergeant?

First and foremost, your soldiers come first. Period.

Secondly....you're a Seargant not an E-5.

I would tell that Soldier to act like a SGT and not an E5.

As an NCO your prior peers are now you subordinates, act accordingly and realize they are no longer your friends, business is business. Be a leader and set a good example, know the standards and live by them. The Soldiers follow NCOs more if the NCOs do not mind getting a little dirty with their troops. Soldiers and mission is important, there will be times where you have to make tough decisions between mission and Soldiers. Be able to stand by your decisions and the consequences or rewards that come with them. There will be a lot of late nights you will have to work and weekends. Be tactically and technically knowledgeable and ensure you can train Soldiers. Good luck.