the viewing situation can have a significant effect on scariness...
try watching the chosen film at night (preferably winter!), in the dark (a power cut might be nice but might cause problems!), alone in your house/flat (with no-one coming back later in the evening to talk to!) then try to go to sleep afterwards (which will probably force you to start contemplating what you have seen rather than being quickly distracted by something else!).
i find the 'scariest' films are ones that are unsettling rather than overtly scary e.g. the Ring, Blair Witch, Exorcist, Texas Chainsaw Masacre, giving you something unpleasant to think about rather than some quickly forgetten 'jump-out-of-your-seat' frights.
Dan