For Eclipse
A good way to add external JARs to your Android project or any Java project is:
libs
in your project's root folderlibs
folderNow right click on the Jar file and then select Build Path > Add to Build Path, which will create a folder called 'Referenced Libraries' within your project
By doing this, you will not lose your libraries that are being referenced on your hard drive whenever you transfer your project to another computer.
For Android Studio
Please Note : If you are using 3rd party libraries then it is better to use transitive dependencies where Gradle script automatically downloads the JAR and the dependency JAR when gradle script run.
Ex : compile 'com.google.android.gms:play-services-ads:9.4.0'
Read more about Gradle Dependency Mangement