Migrating Large Database from SQL Server 2008 to 2014
I'd suggest setting up database mirroring.  

Assuming you setup mirroring in synchronous, high-safery, mode, the 2008 server will send data over to the 2014 server, as it is modified in the source database.

Once you decide to cut over to the new machine, you simply initiate a failover to the mirror database, "et voila", after a minute or two required for the 2014 machine to "upgrade" the database, it will come online and be ready to use.