Image source: Getty Images. I would like to receive emails from you about product information and offers from The Fool and its business partners. Each of these emails will provide a link to unsubscribe from future emails. More information about how The Fool collects, stores, and handles personal data is available in its Privacy Statement. Peter Stephens | Thursday, 4th June, 2020 Investing £500, or any other amount, in a Stocks and Shares ISA on a regular basis may seem unwise after the FTSE 100’s recent market crash. Although the index has rebounded from its first quarter lows, it continues to trade around 20% down on its price level from the start of 2020.Moreover, the index faces a highly uncertain period. And that could cause many investors to decide on lower-risk opportunities, such as a Cash ISA.5G is here – and shares of this ‘sleeping giant’ could be a great way for you to potentially profit!According to one leading industry firm, the 5G boom could create a global industry worth US$12.3 TRILLION out of thin air…And if you click here we’ll show you something that could be key to unlocking 5G’s full potential…However, by investing regularly in high-quality FTSE 100 stocks over the long run in a tax-efficient account such as an ISA, you could significantly improve your financial outlook.FTSE 100 diversificationPerhaps the most important step to take when buying FTSE 100 shares is to diversify. At present, it’s difficult to know exactly what the post-coronavirus economy will look like. Consumer habits may have changed forever. Or they may quickly return to their previous trends.Therefore, investors should buy a range of companies that operate in multiple industries. That way they can reduce their risk of buying companies that have outdated business models.Likewise, buying stocks that operate in different economies could be a shrewd move. Some countries may be hit less hard by coronavirus than others. That could mean having exposure to a range of regions helps to reduce your overall risks and improves your long-term returns.Financially-sound businessesInvesting in FTSE 100 companies with strong balance sheets and solid market positions could enable you to capitalise on the index’s likely rebound. They may be well placed to survive a period of lower sales and profitability. That could help them to extend their market positions at the expense of weaker rivals.Through analysing company accounts and assessing areas, such as debt levels, interest cover and free cash flow, it’s possible to determine which businesses may be relatively strong. They may trade on premium valuations as a result of their relatively stable outlooks. But paying a higher price for a quality business could be worthwhile in the long run, due to its capacity to deliver higher returns.A long-term outlookInvesting regularly in FTSE 100 shares can be a disheartening experience if they experience difficult periods. With the world economy facing a major recession, it would be unsurprising for investors to experience paper losses in the short run.However, maintaining a regular investment during such periods can improve your returns. It allows you to buy stocks while they trade on even lower valuations. And that can lead to higher capital growth over the long run.The FTSE 100 has always recovered from its downturns to post new record highs. So adopting a long-term outlook for your portfolio’s returns could enable you to more easily capitalise on challenging periods.History has shown that they’ve usually been the most logical times to buy high-quality businesses. I’m sure you’ll agree that’s quite the statement from Motley Fool Co-Founder Tom Gardner.But since our US analyst team first recommended shares in this unique tech stock back in 2016, the value has soared.What’s more, we firmly believe there’s still plenty of upside in its future. In fact, even throughout the current coronavirus crisis, its performance has been beating Wall St expectations.And right now, we’re giving you a chance to discover exactly what has got our analysts all fired up about this niche industry phenomenon, in our FREE special report, A Top US Share From The Motley Fool. Enter Your Email Address Click here to claim your copy now — and we’ll tell you the name of this Top US Share… free of charge! Our 6 ‘Best Buys Now’ Shares How I’d invest £500 a month in an ISA from today to capitalise on the FTSE 100’s rebound Renowned stock-picker Mark Rogers and his analyst team at The Motley Fool UK have named 6 shares that they believe UK investors should consider buying NOW.So if you’re looking for more stock ideas to try and best position your portfolio today, then it might be a good day for you. Because we’re offering a full 33% off your first year of membership to our flagship share-tipping service, backed by our ‘no quibbles’ 30-day subscription fee refund guarantee. See all posts by Peter Stephens Peter Stephens has no position in any of the shares mentioned. The Motley Fool UK has no position in any of the shares mentioned. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. 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