Give half-order-of-magnitude estimates as confidence intervals. Avoid using "hours" or "days" as estimates. Story points work really well here.
Be extra clear on priorities and burndowns to make it clear that you're not just blowing them off. Give short but frequent demos (not just reports) of progress. If later they're concerned about progress, you can point back to all the times you reviewed the product, touched base about priorities, and agreed on next steps.
Make risk really clear to them. If the project is 'get it done in three months or bust', then the payoff (ten times the principal?) better be high enough to account for the risk (30%? 50%? 70%) that you won't make it on time and on budget. That is, you don't want to wait until failure to discover a bad business plan.
At the end of the day, you need to be willing to be patient with and/or walk away from 'business people' that can't wrap their heads around the fact that 20% profit on a venture with a 50% failure rate is a bad business plan. Making employees stressed or overworked to compensate is not a humane solution to the problem. To that end, don't work crazy hours to meet a deadline. That is bad for you, bad for your team, and bad for management since it enables dysfunctional planning.