How do I efficiently lower my karma?

tl;dr - The most evil things you can do:

1. Take the Devil's Highway Perk (-2000 karma)
2. Blow up Megaton (-1000 karma)
3. Blow up the Citadel (-1000 karma)
4. Infect Project Purity (-1000 karma)

You'll notice none of those things are repeatable, however. If we use "donating purified water" as our baseline (+50 karma), repeatable actions that are at least as evil as giving away water is selfless:

1. Giving away FEV modified purified water (-50)
2. Giving Psycho to Paulie Cantelli in Rivet City (-100)
3. Enslaving any character with the Mesmetron: (-100)
4. Killing a non-evil character: (-100)

This wiki page lists all actions you can take to affect your karma. The full list of negative karma actions and their associated karmic impact:

• Devour a corpse (Cannibal perk required): -1 Karma per corpse.
• Stealing from a non-evil character or faction: -5 Karma per instance (note that when stealing from owned (red text) containers, such as cabinets or lockers, the Karma change applies once per activation of that container. Meaning, if you open a container once and steal two items, you lose 5 Karma, but if you open the same container twice, stealing one item each time, you will lose 5 Karma each time, which means you lose 10 Karma). Items can be replaced in owned containers after being stolen, and then stolen again and again with the same Karma loss each time, making for a useful method by which console gamers can decrease their Karma, although repeatedly hacking an owned terminal is a faster way of doing this.
• Repeatedly hacking an owned (red text) terminal (Classified as stealing, but can be done over and over again) -5 Karma per hack attempt, NOTE, even accessing the terminal will result in negative Karma.
• Selling ears taken from the corpses of good characters (you killed) to Daniel Littlehorn via the Contract Killer perk: -10 Karma per ear.
• Killing a good creature: -25 Karma
• Giving away Aqua Pura with the modified FEV with Broken Steel installed, to beggar characters like Carlos outside of Rivet City: -50 Karma per item.
• Performing an evil action in a quest: At least -50 Karma
• Killing a non-evil character: -100 Karma (although, this can be gotten around by shooting them once, making them hostile, then letting a follower kill them.)(note)
• Giving Psycho to Paulie Cantelli in Rivet City: -100 Karma each time so long as he's alive.
• Enslaving any character with the Mesmetron: -100 Karma
• Causing Mister Lopez to commit suicide (either by pushing him or insulting him): -500 Karma
• Letting the feral ghouls into Tenpenny Tower in the Tenpenny Tower quest: -600 Karma
• Blowing up Megaton in The Power of the Atom quest: -1000 Karma
• Infecting Project Purity with the modified FEV in the Quest Take it Back!: -1000 Karma
• Blowing up the Citadel in Who Dares Wins in the Broken Steel add-on: -1000 Karma
• Taking the Devil's Highway perk in the Broken Steel add-on: -2000 Karma (essentially, it resets your Karma to the minimum -1000).

By far the simplest way to acquire Bad Karma in Fallout 3, provided you have the Broken Steel add-on installed, is to take the Devil's Highway perk. It instantly sets your karma to the bare minimum of -2000. ...However, it can only be taken starting level 24, so it's not exactly an option for early-game karma farming.

Some simpler, repeatable options for karma loss include:

• Use of the Mesmetron. Nameless NPCs can be enslaved for -100 karma a pop, regardless of the enslaved character's karma (ie, enslaving raiders is considered just as evil as enslaving innocent civilians). This is normally a bit dicey because of the 50% chance they'll either attack you or explode and the need to replenish your collar supply with each victim, but it can actually be farmed fairly easily by a method described in the wiki page for the collars. This is probably the easiest way to go about farming bad karma provided you're in a position to progress the Paradise Falls questline far enough to get the Mesmetron and some collars:

It is possible to turn hostile unnamed non-player characters (e.g. raiders) into friendly characters by first enslaving them and then instantly removing the slave collar. A Science skill of 75 or higher is recommended. Afterwards the player will again have the slave collar in their inventory, which can be used to enslave other non-player characters.

This procedure however, results in a lot of bad karma for the player. It is an effective means to accumulate bad karma without actually being bad. The removal of the slave collar does not reward positive karma if the player has enslaved the character. [Source]

• Theft. Each discrete 'theft' is -5 karma (so, stealing five owned items out of a container at once is -5 karma, but opening an owned box 5 times and stealing an item out of it each time is -25 karma). Items stolen from owned containers can be put back and stolen again for even more karma loss, resulting in a somewhat tedious but very simple way of farming bad karma.
• Paulie Canelli. This Rivet City trader will beg Psycho off you, and for each you give him you'll get a -100 karma hit. Note that he will die eventually if you give him even one dose, but if you stuff your pockets full of Psycho and gift him as much as you can before he keels over, you can get a pretty hefty karma loss for the price of one (very minor) named NPC's life.
• Cannibal perk. Only -1 karma per corpse guzzled, so it's not such a good primary method, but you're probably going to be killing a lot of people in this game anyway so it's a decent perk to take if you want to make sure you always have a way to drop your karma just that little bit more. (Plus, it just feels evil.)

The FO3 wiki page on karma has a list of many more ways to get negative karma in the game if you're looking for more options, but many of the ones listed there are either one-time-only or involve a lot of murdering of named NPCs.