There's two or three real ways to do it as discussed in the Admin Guide. The "best" scenario depends on your restrictions or needs.
I would normally start with the Locate - Collect - Migrate (Server-Driven) migration, in which the ev service account or another account you choose logs on to each computer with enough access to hit remote registry and disks (admin access). Once you've got a list of computers and files, you can choose which ones to migrate. It really helps if you get all your PSTs out on a network share first...
The Client-Driven Migration relies on the enterprise vault client to find the files. The client then sends chunks of PST files across the network to the PST Temp folder. From there the chunk goes to the PST Holding folder and is migrated into the associated mailbox. You'd only want to enable a few clients for client-driven migration at a time.
Client-Driven Migration can be enabled by right clicking Personal Store Management and selecting from the list of provisioned users.
Of course there are a couple of other options -
As an administrator you can manually import PST files by collecting them locally and then using the Admin Console to import them. You might want to think about doing any PST files which are for terminated users, etc, before you do the rest of the users. This is because EV tends to base its determination of who gets the items in the PST on who had the PST mounted in their Outlook profile....
You can also educate users to mount their PSTs and choose to import items into their archive. The drawback here is that you can quickly get into a capacity situation with too many people asking for too much at once.
Note: Recently with EV 2007 SP4 I have been having some trouble with client-driven migration
