LegalJob recently received the following question (which has been asked before in different ways): 

Q:  Other than great grades, can you tell me a couple of items to work on that will make me more attractive to the big firm?

Answer:  Practical experience that involves some “heavy lifting” and very strong writing skills (preferably not demonstrated solely by your winning moot court brief, although that is nice to have).
 
Real world experience.
Find a place that will allow you to work on substantive matters and contribute in a meaningful way.  Big firms (and future employers) should be able to get a good sense of your experience from reading your resume (and perhaps a short chat with you).  If it is not clear that you functioned as full-time employee (albeit a junior one), it may not constitute heavy lifting.  Put yourself in the future firm’s perspective.  The firm wants one who has demonstrated that he or she can add value immediately.  Your contribution may result from knowing how to perform certain tasks efficiently or knowing and understanding the main legal issues faced by the firm’s biggest clients.

Researching, working on file memos, and working on case books and articles authored by partners are good but try to mix with some other activities that may suggest deeper involvement such as developing solutions to client problems (with partners, clients, or even alone), interfacing with clients, helping draft transactional documents, helping draft briefs, helping prepare for depositions or meetings (even if you are not speaking), and preparing correspondence to the client (which could include memos or even e-mails), etc.

Strong writing.
The perfect job combination before the big firm job would be one that provides hands-on experience and plenty of opportunities to write and improve your writing.  Law schools have legal research and writing class but not many other opportunities to work on this crucial skill.  Big firms expect you to be good at writing and to be able to write succinctly and clearly and to do so in short order (both in terms of coming to the firm and the amount of time on any one assignment).  In addition to legal documents such as briefs and transactional documents, you will be required to write memos to firm partners (and perhaps clients, even if indirectly) which provide recommendations based on the facts and the law and e-mails to firm partners (and perhaps clients, even if indirectly) which may provide recommendations. 

In addition, the prospective law firm will likely be able to assess your writing ability by your cover letter and other correspondence with the firm.  Given these realities, it may make sense to take writing classes outside of law school so that you have something tangible to bring to the firm in this area (and many of your competitors with similar writing ability will not likely have done this). 

Take such a class or classes and make a strong first impression with your clear and concise writing.