If you spend any time at all at the command prompt in OSX or other Linux-ish systems, tmux is a must. Basically it gives you persistent shell sessions, once using it you’ll wonder how you ever got by without it…
For OSX the easiest way to get tmux is via (the also excellent) Homebrew. After installing Homebrew, just do a
brew install tmux
from a OSX prompt (you ARE using iTerm2, right? …Rather than OSX’s rather horrible built-in Terminal app…?)
Add to the mix the nice Bash replacement Fish, which gives you all sorts of command line goodness (color coding, auto-completion, …). Very nice!