Actually a variant on GLOCs reccomendation.
V-Planner (other dive planning software also available
), printer, laminator and a bottom timer. Print and laminate tables and stick to a wrist slate or scrawl in your wetnotes (or any other way that you can think of to attach them to yourself).
After a while you will have a "library" of tables that you can select from. Plus by playing with the planning software you can start to recognise patters that appear in the deco schedules.
Also when you have bought a computer to suit your planned diving you can still take your tables with you - just in case your computer fails.