First things first:
Read the mod's instructions. ALL OF IT. Bothering with anyone who has ignored the instructions is an utter and complete waste of time. Make sure you have the required mods installed, ALL OF THEM, and make sure your minecraft worked with those installed.
Just because your minecraft reaches the menu screen doesn't mean it works. Create a new World, run around a few steps, mine a Block. If it hasn't crashed yet, you probably did not mess up.
Common mistakes resulting in blackscreens:
Did you delete the META-INF folder in your client minecraft.jar? Leaving it intact crashes 100% of the time. In your minecraft_server.jar, the same folder needs to stay.
In case of new Minecraft version, delete the following folders (if present): mods, config, buildcraft, redpower. These folders may contain outdated files being loaded still, and crashing.
Did you make sure you have the correct versions of ModLoader and ModLoaderMP, if needed? Redownload the latest ones if you are unsure.
If in doubt, always start with a fresh minecraft.jar
mcpatcher mods do not need to be extracted for gods sake. Put them into .minecraft/mcpatcher-mods/ and NOTHING ELSE.
I'm getting a black screen and i followed all instructions?! What now?:
Go to https://github.com/medsouz/MinecraftError and download the Utility from there. It will get you a crashing error.
Before you run off and spam the board with your error on pastebin, take the time to READ IT. The crashing reason is usually stated in more or less clear english and if it's something obvious you might not need help to fix it.
Mod specific issues
Minions: YogBox - no Deeds. The YogBox disables the Minion Deeds on purpose. The Master Staff can only be obtained by other means, see YogBox description for details.