Bringing Opportunities to Our Small Business Taxpayers Act or BOOST Act - Amends the Internal Revenue Code to: (1) make permanent the $100,000 expensing allowance for small business assets; (2) expand eligibility of small businesses for cash accounting; (3) revise S corporation rules relating to built-in gains, passive income, and shareholder eligibility; (4) allow certain small businesses a tax credit for contributions to employee retirement plans; and (5) allow a deduction from self-employment income for retirement plan contributions and for health insurance costs.