Learn the Jakeiho transitions from her strings: (Inputted usually with 2P+K/8P+K)
PPPP~JAK
PKK~JAK
6PP~JAK
KK~JAK
6K~JAK
3K~JAK
And any variation of her "rapid poke" attacks usually inputted by holding P and inputting either (2) or (8).
PP4PPP~JAK
3PPP~JAK
66PPP~JAK
BT 2PPP~JAK
Learn the different variations on pokes like her PPPPP string such as:
PPPP~JAK
PP2PP
PP2KP
PP2KT
PP4PPP
PPKK
Or any variation of these such as doing just PP2K.
Kasumi's High/Mid/Low Mix-up game is better than Christie's, though Christie can control space better and is more dangerous in a corner with her 66T, 3P+K~T, and various wall-slam moves such as 6KK, 3K, 7K, JAK~6P+K, JAK~6P, KK, etc. Your goal with Christie is to find a way to back an opponent into a corner and to keep them there. Her 5P & 6P strings can do an excellent job of keeping an opponent on the defensive. Once you've got the opponent blocking: In open space, you can use 4T to maintain pressure (or 66T/63214T for damage), or 66T when an opponent is backed into a corner.
There's obviously a lot more to her, so this here can help out too:
http://www.freestepdodge.com/threads/christie-as-of-doa5lr-version-1-01.5267/#post-246581
Good luck!