For a smooth transition two things are vital:

1. You *need* an second domain. One that points to the same data, but won’t hurt you if it’s down for a few days.
2. You *need* a web provider that allows you to configure things as soon as you initiated the transfer.

With the help of the second domain you can test the transfer without haste, because customers won’t notice. Set up the new server and take your time to test everything. Keep in mind that scripts may behave differently, because of:

• Different software versions (of Perl, PHP, MySql, etc.)
• Different settings for sending automated mails
• Different handling for protected directories
• Different handling of write-access for temporary local files for scripts

Don’t forget to test subdomains and invisible features (like automated updates for your customers). If everything’s working, make some preparations against problems

• Set up a hint on your contact page that there may be temporary problems
• Check the e-mail on your website. If possible change it to an address on some other server (or to the second to domain) to make sure it’s always accessible.
• Create a hidden page that only exists on the new server. This allows you to check if the transfer was done.

If you’re ready start the transfer. Immediately start configuring your new hosting account and set up the domain’s root folder and create the required e-mail addresses.
Okay, time to relax, wait and check the site every now and then. However, then transfer times for domains can very a lot, for example:

• Some TLDs transfer much faster than others (e.g. .de is faster than .com)
• If you start transferring multiple domains at the same time, they may arrive quite differently (many hours in fact)
• Different nameservers update at different times. Maybe the domain isn’t transferred for you – but for your customers it already is.

I hope this helps you a bit to get a smooth transition to your new server.