Ask HN: How to avoid IRS issues with 1099 workers?

Many "on-demand" startups seem to have issues with lawsuits about employee classification (e.g. Uber and HomeJoy) when they use 1099 contractors instead of employees. Is there a surefire way to avoid these issues when using 1099 contractors? Do these issues only occur when you deal with individual contractors? For example, if you build an Uber for plumbers but only dealt with plumbing companies and not individual plumbers on 1099 there wouldn't be any mistake, since the company couldn't be an employee, right? What about giving people a choice between W2 and 1099...does this still come around to haunt companies if workers choose 1099?