if you're using JarSplice do as follows
1) only export your project from eclipse to a jar (i.e. the project class files and resources such as images, sounds, etc), no need to add any 3 party libraries to it at this point.
2) Start JarSplice, on the jars tab you will add your exported jar, lwjgl.jar and any other external jars your project relies on.
3) On the natives tab add all the lwjgl natives
4) on the class tab add the name of your Main class (complete with any package names).
5) Then just click build to create an executable jar.
"Exception in thread "main" java.lang.NoClassDefFoundError: org/lwjgl/LWJGLException"
The above exception usually means that lwjgl.jar is missing.