For example I use the following now:

log_format  keyvalue  'time=$time_iso8601 ip=$remote_addr user=$remote_user req="$request" '
                  'status=$status bytes_sent=$body_bytes_sent req_time=$request_time ref="$http_referer" '
                  'ua="$http_user_agent" forwarded="$http_x_forwarded_for"';

Put this in your http {} block and wherever you define a new access log file append keyvalue (or the name you have chosen) like this:

 access_log  /var/log/nginx/access.log  keyvalue;

And voilà:

 $ tail -1 /var/log/nginx/access.log

time=2014-04-15T18:20:54+02:00 ip=127.0.0.1 user=- req="GET / HTTP/1.1" status=301 bytes_sent=184 req_time=0.000 ref="-" ua="curl/7.36.0" forwarded="-"

If you want to know which variables are available, read the nginx docs.