Minecraft wiki entry for Texture_Pack, associated text: Texture packs may redesign only some textures. So if the main menu looks the same after a pack is selected, it doesn't mean the pack is not working.
Many texture packs may be installed and overlap each other. The texture pack list can be scrolled by dragging the scrollbars up or down.
Minecraft may lock the currently used texture pack (for example, if the pack contains custom textures for user interface), so the file can't be overwritten. If the pack needs to be updated, it may be necessary to temporarily switch to the default pack and then overwrite the file.
To get an unzipped directory to show as a texture pack in Minecraft, that directory needs a pack.txt file in it. This, however, did not work prior to 12w23a.
Keep in mind that, if a texture pack is downloaded in ZIP form, it may contain another folder inside of it that has the texture pack's title, this is the actual texture pack. In this case, this folder must moved to the texture packs folder.
If an older version of Minecraft (1.2.5 for example) is being used and a texture pack for a newer version is installed, the texture pack still runs properly, and ignores the unused items or blocks.
If an older texture pack is used in a more recent version of Minecraft, then the newer blocks and items show a "missing texture" because the texture pack isn't made for newer versions.
An editor is a great way to make a texture pack.