With most of us trading stocks online now, checking the performance of your portfolio is easy enough. However, there's another portfolio you should also check out once in awhile, even though it takes just a bit more work to set up. Because with this other portfolio, you can learn how to better invest -- or divest -- for optimum returns.

But before describing how to go about setting up this other portfolio, or anti-portfolio, let's discuss one of the more famous anti-portfolios run by Bessemer Venture Partners, or BVP.

A long history of missing fantastic investments
In 1901, J. P. Morgan bought out Carnegie Steel for $480 million, allowing him to create United States Steel Corporation (X -2.13%). Henry Phipps, Jr., the lesser-known co-founder of Carnegie Steel, took his share of proceeds and created BVP to begin investing in other entrepreneurs. Since then, BVP has been a part of 108 IPOs and invested in newer companies like LinkedIn (LNKD.DL), Skype, and Staples (SPLS), as well as old-timers including the company that eventually became Intel (INTC -5.16%).

BVP is extremely successful at picking winners. For example, its $12.8 million investment in LinkedIn was worth $430 million after LinkedIn had its IPO. And since that IPO in 2011, LinkedIn has more than quadrupled its revenue per quarter, with $447 million in the last quarter of its fiscal 2013. LinkedIn has also grown its membership from about 100 million in 2011 to 277 million today. BVP was able to identify its potential extremely early, investing in LinkedIn in 2006.

But while BVP has been a part of many fantastic investments, it has screwed up spectacularly as well -- and it openly admits this in its own anti-portfolio. This portfolio of could-have-beens includes early investment opportunities in Apple (AAPL 0.86%), eBay (EBAY -1.12%)FedEx (FDX -1.61%), and Google (GOOGL -1.05%), all of which BVP passed on. As BVP partner David Cowan said of eBay: "Stamps? Coins? Comic books? You've got to be kidding, no-brainer pass."

By keeping this anti-portfolio, BVP keeps itself grounded. And BVP can look back on these mistakes, notice trends, alter its strategy, and learn from all of this in order to perform better in the future.

Your own anti-portfolio
Investing in stocks is a bit different from venture capital, so an individual investor's anti-portfolio won't be filled with missed opportunities like BVP's -- otherwise, that might include the several thousand stocks that can be publicly traded. You probably already have a watchlist of stocks, alerting you to changes of what you could own.

For an individual investor, the real missed opportunity is selling before you should. How can you fine-tune your selling behavior? Simple.

Every time you sell a stock, create a holding in a portfolio of the same amount and price that you've sold (there are plenty of free online portfolio services). Then, look back on this portfolio once a month or so. If these holdings trend positive above a benchmark like the market's performance, you've likely sold too soon. If this happens, check your reasoning why you sold that stock, and alter your future behavior to attempt to fix that mistake. On the other hand, if these holdings perform worse than your benchmark or the market, then congratulations! You sold at the right time. And even though you made the right decision, still go back and check your reasoning to see what you might want to repeat in the future.

Improving your investing
Many people just throw money in and out of stocks without tracking their reasoning or performance outside of what gets automatically tracked by their brokerage. They score some wins, and suffer some losses, without taking any lessons from either. Take a page from the century-old BVP, and improve your investing skills.