PC Cheats:

All weapons:
1 - Light a flare
2 - One step forward
3 - One step back
4 - Turn around three times in either direction
5 - Back flip

Skip level:
1 - Light a flare
2 - One step forward
3 - One step back
4 - Turn around three times in either direction
5 - Jump forward

PS1 Cheats:

All weapons (PAL):
1 - Light a flare
2 - Step forward
3 - Step back
4 - Turn around three times in either direction
5 - Back flip

All weapons (NTSC):
1 - Sidestep left
2 - Sidestep right
3 - Sidestep left
4 - Step back
5 - Step forward
6 - Turn around three times in either direction
7 - Back flip and roll in mid-air

Skip level (PAL):
1 - Light a flare
2 - Step forward
3 - Step back
4 - Turn around three times in either direction
5 - Jump forward

Skip level (NTSC):
1 - Sidestep left
2 - Sidestep right
3 - Sidestep left
4 - Step back
5 - Step forward
6 - Turn around three times in either direction
7 - Jump forward and roll in mid-air