When restoring a backup, how do I disconnect all active connections?

You want to set your db to single user mode, do the restore, then set it back to multiuser:

ALTER DATABASE YourDB
SET SINGLE_USER WITH
ROLLBACK AFTER 60 --this will give your current connections 60 seconds to complete

--Do Actual Restore
RESTORE DATABASE YourDB
FROM DISK = 'D:\BackUp\YourBaackUpFile.bak'
WITH MOVE 'YourMDFLogicalName' TO 'D:\Data\YourMDFFile.mdf',
MOVE 'YourLDFLogicalName' TO 'D:\Data\YourLDFFile.ldf'

/*If there is no error in statement before database will be in multiuser
mode.  If error occurs please execute following command it will convert
database in multi user.*/
ALTER DATABASE YourDB SET MULTI_USER
GO

Reference : Pinal Dave (http://blog.SQLAuthority.com)

Official reference: https://msdn.microsoft.com/en-us/library/ms345598.aspx

SQL Server Management Studio 2005

When you right click on a database and click Tasks and then click Detach Database, it brings up a dialog with the active connections.

By clicking on the hyperlink under "Messages" you can kill the active connections.

You can then kill those connections without detaching the database.

More information here.

SQL Server Management Studio 2008

The interface has changed for SQL Server Management studio 2008, here are the steps (via: Tim Leung)

1. Right-click the server in Object Explorer and select 'Activity Monitor'.
2. When this opens, expand the Processes group.
3. Now use the drop-down to filter the results by database name.
4. Kill off the server connections by selecting the right-click 'Kill Process' option.

I ran across this problem while automating a restore proccess in SQL Server 2008. My (successfull) approach was a mix of two of the answers provided.

First, I run across all the connections of said database, and kill them.

DECLARE @SPID int = (SELECT TOP 1 SPID FROM sys.sysprocess WHERE dbid = db_id('dbName'))
While @spid Is Not Null
Begin
        Execute ('Kill ' + @spid)
        Select @spid = top 1 spid from master.dbo.sysprocesses
        where dbid = db_id('dbName')
End

Then, I set the database to a single_user mode

ALTER DATABASE dbName SET SINGLE_USER

Then, I run the restore...

RESTORE DATABASE and whatnot

Kill the connections again

(same query as above)

And set the database back to multi_user.

ALTER DATABASE dbName SET MULTI_USER

This way, I ensure that there are no connections holding up the database before setting to single mode, since the former will freeze if there are.

I prefer to do like this,

alter database set offline with rollback immediate

and then restore your database. after that,

alter database set online with rollback immediate