Playing in the Sandbox...
1) never railroad your players
2) give a hook for the players to follow and keep them following it.
3) be ready to name an NPC ( a list of names help)
4) be ready to describe details of something the players are encountering.
5) let the group talk amongst themselves and roleplay if you get stuck and need to think a minute.
6) taverns or inns are the perfect place to play in, as you can always make up something from a fight, to someone stealing something or as simple as there is a ton of rats in the basement.
7) for dungeons and describing rooms... think of various places you've gone or think of scary movies, and the places people run into that killer.
8) steal ideas from other DMs. There is a buttload of actual play podcasts out there, listen to a few (if you can tolerate the bad audio), jott down a few things and twist it with your warped thinking.
We'll talk more about this in the future....