That's a typical situation that you can avoid easily following some best practices. I suggest you to read Venture Hacks and AVC's MBA Mondays that have covered this topic in detail.
Shares must never be assigned upfront. Every founder will vest his share over 3 or 4 years.
Reserve a share pool for consultants, advisors, future partners. You don't need to assign all the shares, reserve 30% of them for future use. You can even use them to further compensate cofounders who did exceptional work.
Set clear deliverables. It must be very clear what every cofounder or employee is supposed to do, and what results he's supposed to bring. That's particularly true for non-technical founders (business development, sales, marketing) that often refuse the idea to have clear deliverables; trying to sell, or trying to attract customers is not a deliverable: sell 100 licenses over a month, set up 50 customer meetings in 6 months, attract 1000 signups per day are clear deliverables.
Once you have clear deliverables, it's easier to prove/disprove if a cofounder/employee is performing adequately.