Easy combos:
Get the chainsaw, and the combos will become easier. Stand over a Manhole, let
the chainsaw rip, and watch your combos, go over the top. Ones with lots of
enemies around them are the best, you can get up to a 10 combo with
one slice.

Unlock Santa Claus and Bloodrayne:
Finish  all missions.

Getting avatars:
Kill the zombies near  the slipknot sign in the following missions:

Slipknot 0 - Sign is located at Time Square in mission 5.
Slipknot 1 - Sign is located in the subway in mission 7.
Slipknot 2 - Sign is located by the helicopter  in mission 10.
Slipknot 3 - Sign is located in the upper left section of the map in mission 13,
Slipknot 4 - Sign is located near the landing zone In mission 16.
Slipknot 5 - Sign is located in Time Square in mission 20.
Slipknot 6 - Sign is located in the bottom section of the map in mission 23.
Slipknot 7 - Sign can be found when the missionstart screen in mission 26.
Slipknot 8 - Sign is located in the bottom right section of the map in mission 31
Chimaira - Sign is located in the long street section under the park in mission 29

Get the $1 Avatar:
Enter the code when examining the $1 Avatar -
Use the R toggle him to spin. Spin him twice on the second spin stop so that he is
looking at his back. Then press Triangle X,X then Triangle Circle