Are you sure you are putting the mods in the right folder and everything?

you need put .class files in %appdata%\.minecraft\bin\minecraft.jar (open it as 7 zip or winrar) delete folder META-INF inside minecraft.jar and put inside .class files

Also, some mods just don't work with other mods, so have a copy of your minecraft.jar.

I suggest you download the Yogbox. It's like a package of mods.

You've gotta delete META-INF folder.

I made a lot of mods review, so here are my advices :

1/ Check the mod version (available for 1.2.5 ?).
2/ Delete the META-INF folder.
3/ Clean up the .minecraft>mods folder.
4/ Don't hesitate to use the Forced update option to get a clear minecraft.jar.
5/ Mods have different way to install. Be sure to check every readme.txt or the installation guide made in the official mod post.
6/ If there is one, check the "Compatibility" section inside the posts.
7/ Check which mods are necessary to run these mods (the main mods are ModLoader, ModLoader MP, Forge, AudioMod, GuiAPI and PlayerAPI, but there still can be others).
8/ Finally, check the version of this installation mods.
9/ Some mods need a powerful computer to be used (I'm thinking about Sonic's Unbelievable Shaders). Look forward this.

After all this, it should work perfectly.

Download "technic launcher"