This is a very brief tutorial on how to use MagicLauncher to load mods like OptiFine into Minecraft so you don't have to manually edit the minecraft.jar file. This should give you one less excuse for resisting the performance-boosting magic of OptiFine.
STEP THE FIRST: Acquire MagicLauncher
http://magiclauncher.com will redirect you (click the title on the page) to the current thread on minecraftforum.net where you can download the MagicLauncher app for your system. You'll find there are Mac and Windows apps, and there is also a raw JAR for Linux users or people who can't get the GUI apps to work. Download the appropriate version and put it somewhere near where you normally keep your copy of Minecraft.
STEP THE SECOND: Acquire OptiFine (or other client-side mods)
http://optifine.net will redirect you (click the title on the page) to the current thread on minecraftforum.net where you can download the latest versions of the OptiFine client-side mod. When you see the first post, don't be paralyzed by the myriad options. If you don't want to read or don't understand the various options, just go with the first version.
Do not expand the downloaded zip file. If you're using Safari, turn off that retarded "open files after downloading" option and try again. Put the downloaded zip file somewhere convenient; I created a "thirdpartymods" folder in my minecraft folder and put it there.
Protip: If you're going to put the downloaded mod in your ~/Library/Application Support/minecraft folder, don't put it in a folder called "mods". This name has a special meaning for some mod-loaders and will be treated differently by MagicLauncher.
STEP THE THIRD: Launch MagicLauncher
Upon first launch, you'll probably see something like this. MagicLauncher has a bunch of Minecraft news/reddit/twitter/etc junk that you can read at launch similar to the official Minecraft launcher. You could just continue to log in at this point, but that would just launch Minecraft without any modifications. So, first click that Setup button.
STEP THE FOURTH: Load OptiFine into MagicLauncher
Once you access the Setup screen, your primary objective is to add the OptiFine zip you downloaded earlier. Click the Add button, and load 'er up.
Verify that the mod's Active flag is set and the Status is OK. When new versions of Minecraft are released, mods have a tendency to break because the points at which the mods inject code get shuffled around (this is a simplification, but go with me here). MagicLauncher sanity checks the mods here and alerts you if it thinks the mod is broken. If that's the case, go see if a new version is available and start back at STEP THE SECOND.
STEP THE FIFTH: Play Minecraft!
When you're done adding OptiFine, hit the OK button to return to the main screen and login to start playing Minecraft.
To access the options added by OptiFine, go to the Video Settings screen. See all the new buttons? And whole new screens of options after you click some of them!
If you hover over an option, you'll also get a brief explanation of what it does and how it affects performance. Note that some options can improve the visuals of Minecraft without a negative impact (such as my setting of Fog Start: 0.8).