This is not an exhaustive list but it is a start. First off, remember that studios don't necessarily care if you watch their film again but they definitely want you to buy their film again... I'm looking at you, copy of Shindler's List just sitting on the shelf after 15 years, still in the original plastic.
OMG Special Features!!!@!
One of the most obvious ways of getting people to re-watch a film is to re-release it, usually with more "special features".
Is your film having a 10th... or 15th, or 25th... (you get my point) anniversary soon?
Put out a new copy of it with commentaries, behind the scenes footage, stills, deleted scenes, etc. You will get fans to buy the special version just for these updated, previously unreleased special stuff.
Release Different Versions:
And, along this vein... let's just release a new version of the film while we're at it... 3 hours of Lord of the Rings not enough for you? How about 3.5 hours??? Extended cuts are popular with fans and studios because the fans get more film and the studio gets a lot more money.
Or, how about implying that the director hated what the editors did to his film, so we need the director to release a special "Director's Cut" of the film.
Or, if you're George Lucas... you just release new versions every time you think that technology and plot have changed enough that you need to release a new version. Poor Star Wars. Shall we ever see you in your original glory again?
Show it in Theaters Again:
Are you having a sequel coming out soon?... perhaps after 20 years? How about getting theaters to screen double features so that people can remember what they forgot 20 years ago right before seeing the new one.
"Vault" Your Films:
This one is really just Disney Animation but they seem to think it works. Disney has been creating artificial scarcity of their films by limiting their DVD/BluRay releases and "vaulting" titles for periods of several years. When the film "comes out of the vault", have a big release event and everyone will buy the copies.