How can you increase the chance your portfolio will last as long as you do? That is, after you retire, how can you feel confident your investments will last at least 30 years? A recent study, using computer simulation, developed this plan:

* Use the right amount of stocks. Somewhere between 40% and 60% of your portfolio should be invested in stocks. Fewer stocks will crimp long-term returns but more stocks increases volatility. The balance of your portfolio should be in high-quality bonds.

* Diversify your stocks. At the least, hold large-cap and small-cap stocks. You probably should include some foreign stocks or foreign stock funds, too. Diversification reduces portfolio risk.

* Tap your portfolio moderately. Assuming you start while you’re in your 60s, start with a 4.5% withdrawal. Take $9,000 from a $200,000 portfolio, for example, or $22,500 from a $500,000 portfolio.

Going forward, you can increase your annual withdrawals to keep up with inflation. Based on historic records, you’ll be virtually assured that your money will last at least 30 years.