How to split a monthly contribution across several assets to pull a portfolio back toward its targets — without selling anything or paying taxes. Every long-term investor hits the same wall: you set a target allocation (say 40% stocks, 25% fixed income, 20% REITs, 15% international), the market moves, and three months later your portfolio is lopsided. The classic advice is to rebalance by selling what went up and buying what went down. The problem: selling triggers taxes, brokerage fee...