Pop into iTunes and choose File --> New Playlist. Now rename it to something logical: I use "iPhone Sync":
At this point, you need to plug in your iPhone if you haven't already done so. It'll pop up the usual summary screen:
Cool. Now click on the "Music" tab and you'll see that you can select "sync playlist". Do that, then select the new iPhone sync playlist you've created, as shown here:
Oh, and one really nice feature of creating this playlist is that you can manage it, adding and deleting songs to your heart's desire, even without your iPhone present. Whatever changes you make are instantly applied to your iPhone music library the very next time you synchronize the device. Suddenly even 8GB might feel small.