I'd recommend finding two or three samples from each "family" of tobacco and learning the hallmarks of each family: Virginia, Burley, Dark Fired, English, Oriental, Balkan, etc. For example, if you smoke enough Virginias, you'll begin to taste the "grassy" or "citrus" notes as compared to the "nutty" or "chocolatey" notes of Burley blends. Just try to get a solid understanding of each type in and of themselves and it will help you when you encounter them as components in more complex blends.
Some recommendations are:
Virginia: Capstan Blue, Dunhill (now Peterson) Flake, Sutliff Virginia Flake
VaPer: Stokkebye Luxury Navy Flake, Haddo's Delight
Burley: Old Joe Krantz, Haunted Bookshop, any of the C&D Burley Flakes, Lane Ltd Ready Rubbed
Dark Fired: Old Dark Fired, Bold Kentucky, Irish Flake
English: Nightcap. 'Nuff said.
Balkan: Balkan Sobranie, Balkan Sasieni, Mountain Camp.
The more blends you smoke, the more you will be able to distinguish between them. Have fun!