In addition to all the standard shareholder agreements (buy-outs, drag-along, tag-along, death, divorce, etc), I'd ask yourself (and your partner) if one of you walked away after a year or two, three, five, etc, how much equity you would expect to retain (from both perspectives). In other words, come up with a vesting strategy.
I'd also suggest you think about what happens if one of you stops working or gets sick and can't be as productive or just isn't very productive to begin with, i.e. the other one ends up doing all the work. Despite the best intentions, personal situations can change how productive people are which isn't fair to the other person. Death and divorce is relatively easy to agree on, but dealing with a slacker is painful.
In general, talk very openly about your visions for the company, what you would like to see the future bring, what your roles will be. If any of the discussion makes you uneasy, don't just brush it off because whatever you feel now will be amplified when the situation actually occurs. Better to fight through it now when you haven't invested your time.