These days disk space is not a huge problem, but on a MacBook Air it can get a little tight once all your applications and media libraries are installed. This dilemma is not helped by OSX 10.5 Leopard's installation bloat. Not only is multiple language support installed by default, but most applications come with both x86 and PowerPC binary files. So if like me you only understand English, removing superfluous languages and binaries will free gigabytes.
Now whilst it is possible to do this task manually it is hardly fun or a good use of one's time. Fortunately there are a few tools out there that can do the job for you, my favourite being Xslimmer. Whilst this application does cost a handful of dollars (US$12.95) it has a great interface and keeps a 'blacklist' of applications that experience issues when they are placed on a diet, for example Skype. Plus if it helps remember that for this money you are reclaiming storage space, so it could be argued $13 is a very small price to pay when your laptop's hard drive cannot be replaced and external USB drives look ugly.
Using Xslimmer is very easy, simply open it and run the Genie command to automatically locate all your applications. Once found Xslimmer will analyse each program to determine what excess fat can be trimmed. This usually results in a 25%-50% reduction in application size. This soon adds up to gigabytes of space on a typical OSX installation. Once analysis is complete you can choose to backup all unnecessary files or just delete them entirely. After this decision is made Xslimmer quietly gets on with the job, which for me resulted in 2.5 gigabytes of storage being freed (approximately US$5.20 per gigabyte).
The only thing to keep in mind is that as software updates and new applications are installed new fat will be introduced to the system. This means if you want to keep your system nice and lean re-runnig Xslimmer every few months is a good idea.