Eclipse 3.5 added support for relative buildpath/classpath items, yet I cannot seem to find a graphical way to add relatively pathed items. The example in new and noteworthy
The actual way to do this is to import the library into your workspace so,
Right Click in Package Explorer -> Import -> File System -> Browse (to the directory containing your jars) -> check the boxes next to the jars you want -> Browse to the workspace folder you want to import to (probably your lib folder) -> Finish
Keep in mind this will copy the files into the lib folder so I've found that it helps keep things clean if you don't already have them in there. They will now show up in the package explorer under "lib." To add them to the classpath,
Right click on the project -> Build Path -> configure build path... -> Add JARs -> navigate to the given project's lib folder and select the appropriate JARs -> click ok -> click finish
This adds them to the CP with relative locations.
If you need more help let me know