Basic setup of Emacs server under OSX

To make Emacs.app open files in an existing frame instead of a new frame, add (setq ns-pop-up-frames nil) to a configuration file like ~/.emacs.

You can open a file from a terminal with open -a emacs file or emacsclient -n file. If Emacs.app is not open but there is an Emacs --daemon process, for some reason emacsclient -n file doesn't work but emacsclient -nc file does.

Make sure you use the emacsclient binary included with the version of Emacs you use, like /Applications/Emacs.app/Contents/MacOS/bin/emacsclient, or /usr/local/bin/emacsclient if you installed Emacs with Homebrew.

To start an Emacs server at login, for example save this plist as ~/Library/LaunchAgents/my.emacsdaemon.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>my.emacsdaemon</string>
  <key>ProgramArguments</key>
  <array>
    <string>/Applications/Emacs.app/Contents/MacOS/Emacs</string>
    <string>--daemon</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
  <key>KeepAlive</key>
  <true/> <!-- run the program again if it terminates -->
</dict>
</plist>

The plist is loaded automatically the next time you login, but you can load it immediately by running launchctl load ~/Library/LaunchAgents/my.emacsdaemon.plist.

Edit: I still don't know why people are associating file types with an AppleScript application instead of just Emacs.app. The script in kuzzooroo's answer could also be written as a shell script though:

macos=/Applications/Emacs.app/Contents/MacOS
if pgrep -qf 'Emacs.*--daemon'; then
  [[ $($macos/bin/emacsclient -e '(<= 2 (length (visible-frame-list)))') = t ]] && args=-nc || args=-n
else
  $macos/Emacs --daemon
  args=-nc
fi
$macos/bin/emacsclient $args "$@"
open -a /Applications/Emacs.app

You can use Platypus to save the script as an application:

I just started using Emacs, but I have a completely different setup. I made my own Emacs application by making a copy of iTerm.app, changing the CFBundleIdentifier in the Info.plist so that the application uses a different preference file, and setting the default command to /usr/local/bin/emacs. I have added (server-start) to ~/.emacs and I open the custom Emacs application at login. I used Platypus to create an application that runs emacsclient -n "$@";open -b my.emacs and I made it the default application for text files.

Lauri's solution combined with suggestions from the comments have largely addressed my issues. I've pasted below some Applescript that I stitched together from stuff I found online. The script helps smooth out some remaining wrinkles, e.g., its behavior is a little better if there's no current Emacs frame.

EDIT: the purpose of the script is to be associated with .txt files in the Finder.

-- http://superuser.com/questions/457484/how-to-open-emacs-from-macs-finder
-- https://gist.github.com/ambethia/304964#comment-799519
on run {input}
    set filepath to quoted form of POSIX path of input
    tell application "Terminal"
        try
            -- we look for <= 2 because Emacs --daemon seems to always have an entry in visibile-frame-list even if there isn't
            set frameVisible to do shell script "/Applications/Emacs.app/Contents/MacOS/bin/emacsclient -e '(<= 2 (length (visible-frame-list)))'"
            if frameVisible is "t" then
                do shell script "/Applications/Emacs.app/Contents/MacOS/bin/emacsclient -n " & filepath
            else
                -- there is a not a visible frame, launch one
                do shell script "/Applications/Emacs.app/Contents/MacOS/bin/emacsclient -c -n " & filepath
            end if
        on error
            -- daemon is not running, start the daemon and open a frame     
            do shell script "/Applications/Emacs.app/Contents/MacOS/Emacs --daemon"
            do shell script "/Applications/Emacs.app/Contents/MacOS/bin/emacsclient -c -n"
        end try
    end tell

    -- bring the visible frame to the front
    tell application "Emacs" to activate

    return input
end run