Trying to find stocks that you can buy and hold for the long term is a tricky process. Selecting stocks is more of an art than science, and it usually takes investors years to develop the experience necessary to be able to select the best buy-and-forget stocks.

The best stocks to buy and hold for the long-term are usually companies that have sustainable economic moats, economies of scale, throw off cash, or operate in long-term businesses such as insurance.

Aviva (LSE: AV) has two of these four essential qualities. After acquiring Friends Life last year, the company became the largest provider of pensions and savings products in the UK. This means that the group can achieve economies of scale that few other pension providers can manage. For example, in an increasingly competitive pensions market, Aviva can charge investors a lower fee to manage pension funds due to the scale of its operations. 

And the long-term nature Aviva’s business means that the company will continue to earn a fee on its managed funds for decades, great news for long-term shareholders. Shares in Aviva currently trade at a forward P/E of 8.5 and support a dividend yield of 5.5%.

Cash cow 

As well as having a long-term business model, Standard Life (LSE: SL) is also throwing off cash. The company has transformed itself over the past few years into an asset manager that looks after pension assets, collecting a fee for its management services. This means that the group has become highly cash generative as the asset management model is relatively low cost.

Most of Standard’s profits are being returned to investors. The company’s shares currently support a dividend yield of 5.3%, and the payout is expected to increase by around 20% over the next two years leaving the shares yielding 6.1% for 2017.

After a rough start to the year, Standard Life trades at an extremely attractive valuation of 12.1 times forward earnings. City analysts expect Standard’s earnings per share to jump by 99% this year and then a further 10% during 2017. 

Asian exposure 

Considering its exposure to the rapidly growing Asian insurance market, Prudential (LSE: PRU) is another qualifying buy-and-forget insurance stock that would fit well into any investor’s portfolio.

The company reported earnings per share growth of 30% for full-year 2015 and Prudential’s shares are trading at a forward P/E of 11.5 for 2016, which looks cheap when you take into account the group’s historic growth. During the past five years, Prudential’s pre-tax profit has doubled from £1.8bn to £3.5bn. The company’s shares support a dividend yield of 2.7%, and the payout is covered two-and-a-half times by earnings per share. The company generated over £3bn in free cash flow last year according to its full-year 2015 results release, a result which inspired management to declare a special dividend of 10p per share on top of the regular payout.  

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Rupert Hargreaves owns shares of Prudential and Standard Life. The Motley Fool UK has no position in any of the shares mentioned. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.