OK TL
R but here are all the ways I can think of, many have been suggested, but this makes a good compilation:
Kill endermen in the overworld: I find the best way to do this is find a desert (it doesn't rain in deserts and they are usually flat with a large field of view), build a roof at least 3x3 2 blocks above your head and wait for them to spawn at night. Hitting them in the feet won't make them teleport away. Bows or flame/poison enchants will make them teleport, so pick a weapon without these.
Dark Rooms/MOB traps in the overworld: many suggestions on this already and there are a ton of ways to do this. I find a small 10x10 room with 4 MFR Grinders in the middle of an area that's been quarried out to work well enough. Areas that have been mined to bedrock have no caves or places for MOBs to spawn, so they will spawn quicker in a dark room placed in the area.
Cursed Earth: Also suggested many times. Adding Cursed Earth to the above dark room works well.
Grow Enderpearls: Ender Lilies are slow IMO, but can be found in Dungeon Chests (or in The End?) Magic Crops also has an ender plant, although it needs 4 enderpearls to make and is tedious to progress to. There are bees that can be breed to make enderpearls, but this too is tedious to progress to.
MFR Auto-Spawner: You do need 4 pearls to make a Safari Net, but this is the easiest way I know in Monster to get pearls. Placing an Auto-Spawner in the middle of your dark room noted above has the advantage that the dark room/grinders produce MOB Essence to generate the endermen.
The End: Also noted many times, the way to get to the end is to fill an end portal in a stronghold with eyes of ender and dive in. You will need up to 12 ender pearls to make eyes of ender to complete the portal. After getting to The End, you get out vanilla style by killing the Ender Dragon and using the portal that spawns at it's death point. The other good suggestion already made is to make Mystcraft Linking Books and use them. Note that if the Ender Dragon is alive, it can destroy your bookstand, so bring spare Linking Books back home when you go to The End.
Ghetto Enderman Farm in The End: Many good suggestions, many good tutorials exist. I like digging a 1 deep hole with a roof 2 blocks high and when I'm in it, I just look at endermen and slice their feet up.
Vanilla Enderman Farm in The End: Docm77's video on the JL design is very popular, although I prefer my "Enderbowl" design. These farms are easy to build with flight from mods, but they are not the most efficient way to farm endermen.