How to memorize 7 crystal systems?
The first step is understanding what you have.  The seven crystal systems are derived from decreasing symmetry (lifting degeneracies) of the cubic system by altering lengths and angles.  Example: Cube volume is $\ce{a^3}$.  Parallelepiped volume with all symmetries explicit is  

http://en.wikipedia.org/wiki/Parallelepiped


http://www.rockhounds.com/rockshop/xtal/index.shtml 
Good reading about crystal systems and their expression.  
http://en.wikipedia.org/wiki/Space_group 
As usual, nomenclature is a mess. 
http://en.wikipedia.org/wiki/Hermann%E2%80%93Mauguin_notation 
That's a winner.

Crystallographers often translate the unit cell within the lattice to make its contents  "nice." The rhombohedral system is particularly nasty for choosing the unit cell itself.  65 Sohnke space groups of 230 overall contain chiral formula units.  Racemic screw axes make that interesting, ${2_1}$, ${4_2}$, ${6_3}$.  Within the 65, 11 enantiomorphic space groups - themselves chiral without regard to unit cell contents - allow the same substance to crystallize in two mirror-image space groups.