How to Migrate Email from One Gmail Account to Another.

For the last year, I've used a work email address using Google Apps as my main email account. I'm transitioning out of that company now, and want to bring my emails (and GTalk records) with me. Is this possible?
Is there a way to bring my contacts and messages to another Apps account and delete the old stuff? Is there any way at all to do that without losing labels?
Thanks,


Leaving Gmail for Gmail

Dear Leaving,
I'm sure you're not alone—some people simply don't like the username they chose out of the blocks and want to move all of their email to a more professional-sounding email address. Unfortunately, this is one of Gmail's biggest drawbacks—it doesn't let you migrate from one Google account to another with any amount of ease. There are likely a few ways to do it yourself, but if you really want to preserve everything, you're probably best off using a mail client, like Outlook or Thunderbird, to drag and drop the messages between accounts. Here's how it works:
You'll have to set up your desktop client of choice to access both of your Gmail accounts via IMAP. Google details the process for various clients here, so we won't go into every step. Also note that for this to work best, you'll want to enable "Advanced IMAP Features" in Gmail Labs on both accounts, and tell it to show all your labels (including Sent Mail and All Mail) in the mail client on the Labels page in Gmail.

Prepare Your Gmail Account

You're going to be stuck with folders instead of labels in your desktop client, which is kind of a pain, but we can still make it work. If you regularly assign multiple labels to messages in Gmail (keeping in mind that Sent Mail, Inbox, and Starred are also labels), you'll have to do a bit of work to make them more folder-friendly for your desktop client. Probably the best way to handle that is to take all combination of labels you have and assign a new label to it. For example, if you have a lot of messages that are assigned "label 1" and "label 2", create a new label called "labels 1 and 2" that contain all those messages, and then unassign the original two labels.
That way in your folder-focused desktop client, the messages will be in one place and easy to transfer to your new account. When you get the messages to your new account, you can reassign the original labels and delete the third one. This could potentially be a lot of work, depending on how many labels you have and how many combinations of those labels exist in your setup—but it's really the only workaround with this method. You won't have to do this if you just use labels in your Gmail account as you would folders, i.e. archiving all messages that are assigned labels and only assigning one label to a message at a time. Here are some screenshots of the step-by-step process to make it clear:


Multiple-Label Messages:

Create Label Combinations:
 

Remove Multi-Labels:
 

Moving from One Gmail Account to Another from Your Desktop Client

Once your old account is ready for the transfer, you'll have to create all the same labels in your new account that exist in your old one. Then just jump into your client of choice and start dragging and dropping the messages from the old folders to the new folders (by hitting select all in the old folder and then dragging that mass to the new folder). Do this for all your folders/labels, including your Inbox and Sent Mail, excluding All Mail (for now). That should get you a good amount of your messages, but we still need to deal with the un-labeled emails in the All Mail folder.

At this point, make sure all your labeled messages made it safely to your new account, because you'll have to delete them off the old account to migrate the rest of the un-labeled messages. (The deleting is necessary because if you were to copy everything from All Mail to your new account, all your labeled messages would migrate duplicates. If you're queasy about deleting anything, you could also try labeling all your unlabeled mail, but unfortunately Gmail doesn't have a search operator for unlabeled messages, so it would likely be a lot of work.) If everything looks good, go ahead and move all messages in labels, sent mail, and your inbox to the trash and empty the trash (do all this from your desktop client). You should be left with a bunch of empty folders, except for All Mail, which will contain only messages that had been archived but did not have a label assigned to them. Now you can drag these to the All Mail folder on your new account. 
Better Way?
It seems like a roundabout way to do it all, but it's the best method I've found. As far as contacts go, you can easily export those by going to contacts and hitting the "export" link in the upper right hand corner—after which you can re-import them into your new account. Unfortunately, you cannot migrate your chat logs—there just isn't a good way to do it since neither are fetchable by IMAP or POP. You can't even mass forward them with a filter, since Gmail's filters won't forward old mail. You can manually forward the important ones, but that's about it—you'll just have to keep the account open for reference until Google comes up with something better. Good Luck!