No appointment, just walked in and I was seen in 10 minutes. Not bad.

Research the necessary documents for proving identity in your area. 

See if the location you're going to accepts credit cards, or if you'll need cash. 

It looks as though you can renew your licence up to 18 months before it expires, at least in FL. I paid $50 to renew mine this week instead of paying $25 for a replacement license, then paying $50 to renew it in October.

Oh, and take the time to do your hair and makeup. With a little effort and luck, you can take a great licence picture!