Syncing only checked songs in iTunes is as simple as changing a setting and checking some boxes. To do it, begin by syncing your iPhone or iPod to your computer. When the sync has completed, you should see the management screen in the main iTunes windows.
Check the "Sync only checked songs and videos" box
In the center of the management screen, you'll see a box that reads Sync only checked songs and videos. Check this box to turn on the ability to only sync checked items. Once you've done that, be sure to click Apply at the bottom-right corner of iTunes to save the setting and re-sync your device.
Return to Your Music Library
Click Music in the drop-down menu in the top left of iTunes (in iTunes 11 or later) or in the column at the top left of iTunes (iTunes 10 and earlier). This brings you back to your library, where you can choose what songs you don't want to add to your iPhone or iPod.
Next to each song in iTunes is a little checkbox. By default, all songs are checked and thus will be synced to your device. To choose what songs will be synced, you'll need to uncheck them. Go through your music library and uncheck each song you don't want to have synced to your iPhone or iPod.
If you have a large number of items you want to uncheck, there's a shortcut you should know. Begin by selecting all the songs you want to uncheck. If you want to select contiguous items, hold down Shift, click the item at the beginning of the group you want to uncheck, then click the item at the end. All the items in between will be selected, too. To select non-contiguous items, hold down Command on a Mac or Control on a PC and click each item you want.
After you've selected the songs you want to uncheck, right click on them with your mouse (or, using your keyboard, use Control). In the drop-down menu that appears, select Uncheck Selection.
When you've completed unchecking all the songs you don't want, click Sync again. If any of the unchecked songs are already on your device, they'll be removed. You can always add them back by re-checking the box next to the song and syncing again.
Want another technique? Keep reading to learn how to use playlists to do the same thing.