iPod has built in skip protection
iPod's skip protection works because of its memory cache. The hard drive holds the songs from your iTunes Library, but when iPod plays your songs, it copies them from the hard drive into the memory cache, which can hold up to 25 minutes of music (17 minutes for iPod photo). This means between four to seven average songs saved as MP3 or AAC files can fit into the cache at one time. When the cache is full, iPod's hard drive turns off. All the music you hear comes from the cache. When the songs in the cache have all been played, the hard drive turns back on and copies more songs to the cache.
If your songs aren't compressed, in AIFF format, for example, they take up a lot more space and the cache can only hold a few minutes. This means the hard drive has to work a lot harder to keep the cache full. Since it has to turn on a lot more often, the hard disk is more prone to be affected by movement as it loads the cache, and that could make a song skip.
Maximize skip protection
To get the maximum amount of time out your iPod's skip protection, make sure your songs are encoded so their file sizes are small. The more songs iPod can preload into its memory cache, the longer the skip protection. For example: Importing songs as AAC at 128 kilobits per second (kbit/s) instead of at 192 kbit/s increases the amount of songs iPod can store in its cache.
Minimizing how often the hard drive turns on can also increase the amount of skip protection. By not pressing Fast-forward, iPod can play the songs that are already in the memory cache instead of loading new song from the hard drive.
If a song gets skipped over (not played) in a playlist, see "Troubleshooting songs that skip on iPod" for more information.