The only thing you have to do to access your raspberry from internet is the following:

1. register in www.dnsdynamic.org your account and get a domain
2. install in your raspberry the ddclient package
3. configure it with the data of your account

Some details of my configuration (file '/etc/ddclient.conf'):

daemon=60
syslog=yes
mail-failure=PUT-HERE-AN-EMAIL-ADDRESS-WHERE-ALL-FAILURES-WILL-BE-SENT
ssl=yes
protocol=dyndns2
use=web, web=myip.dnsdynamic.com
server=www.dnsdynamic.org
login=PUT-HERE-YOUR-USERNAME-OF-DNSDYNAMIC.ORG
password='PUT-HERE-YOUR-PASSWORD'
PUT-HERE-YOUR-DYNAMIC-WEB-ADDRESS.com

And finally:

# update-rc.d ddclient enable

To add it in the default runlevel.