The well known phrase 'Plan Your Dive - Dive Your Plan' was first coined by Hal Watts, one of the very early cave diving instructors. The typical decompression dive plan consists of several different plans: the main plan that is expected to be followed, plus several contingency plans for emergency scenarios: exceeded planned maximum depth, exceeded planned bottom time, lost deco gas (i.e. decompression will have to be performed on less favorable backup gas), and bail-out or aborted dive (allowing a significantly shorter deco schedule). An important aspect of technical instruction includes learning the skills necessary to prepare these plans.
Good dive plans must balance between logistics concerns (available cylinders, volumes and mixtures) along with depth and time schedules. While desktop dive planning software has been a godsend for preparing technical dive plans, the technical dive computer does not relieve the need to prepare most of these plans before you jump in the water. That's because preparing any mixed gas dive plan includes optimizing gas mixture selections and most importantly breathing gas volume requirements.
Technical divers never rely upon others for backup. While some still take along tables and bottom timer as backup in case their primary computer fails, most technical divers now take a second dive computer. One technique we've noticed is setting the second dive computer for a much less conservative gradient factor such that in an emergency they have a computer that will get them to the surface as fast as possible.