A superbock in TNT is a 4x4 square made of 4 unbroken tetris pieces. There are 43 superblocks known to man:
0000   0000    0000    0000    0000    0000    0000
0000 = 0000 or 1111 or 1000 or 0001 or 0011 or 1100
0000   1111    1111    1110    0111    0011    1100
0000   1111    1111    1111    1111    1111    1111

where
0000 = 1111 or 1122 or 1222 or 1112
0000   2222    1122    1112    1222
(16 here)

and
0000   1332    1122    1233    1123
0000 = 1332 or 1332 or 1223 or 1223
0000   1122    1332    1123    1233
(4 here)

and
0000   1111    1112    1122
000 =  222 or  122 or  112
0      2       2       2
(9 here)
plus appropriate mirror images thereof, another 9

and
0000   2111    1111
00   = 21   or 22
00     22      22
(5 here, if we count mirror images of the first one)
Note that we don't count rotations as distinct, but we do count certain mirror-images distinct (if it is not the case that a rotate will accomplish the same transformation, i.e., the J and L pieces and the 2 S pieces).

The key to having a 600+ Sprint game is to make partial superblocks whose top row is complete. That way, you can stack up to 5 uncompleted superblocks on each side, and fill them in with whatever pieces come. Variety is also key, especially when it comes to using S pieces. Make as many superblocks as you can that have S pieces (there are quite a few, but are non-trivial to make).