Congress recently passed the Mobile Workforce State Income Tax Simplification Act of 2015.
In short, employees would continue to pay income tax on all of their earnings in their state of residence, but would no longer have to pay income tax in a non-resident state in which they worked unless present in the non-resident state for more than 30 days.
Years ago I had a client who was a resident of the District of Columbia and an employee of the UCLA Jet Propulsion Labs. Four times a year he flew to California for quarterly board meetings and his employer was required to withhold California income tax for the time he spent in California, which was less than four days. We had to file a California non-resident tax return to reconcile the withholding with the income he earned attending four Board meetings, that lasted less than eight hours in total.
If this bill the House has passed becomes law, that would no longer happen.