Being a corporate generalist is the best training for going in-house. Patent lawyers can also move in-house with relative ease. Litigators probably have the hardest time making the transition since many law departments continue to outsource their litigation work. If you are a litigator, a real estate lawyer, a tax specialist, or have a narrow area of specialization, then larger law departments are your best prospects.
Having a mix of corporate and employment law experience is very useful since every corporation needs this expertise. If you build good relationships with your clients as outside counsel, then there is ample precedent for non-corporate attorneys assuming the role of GC at a former client company.