The key is to build a diverse portfolio with a mix of different investments that makes sense for your attitude to risk.
A balanced investment portfolio will contain a mix of equities (shares in companies), government and corporate bonds (loans to governments or companies), property and cash.
Here, we give you five things to consider when putting together a diverse portfolio.
Step 1: Choose a range of assets
Having a mix of different asset types will help you spread risk. It’s the old adage of not putting all your eggs in one basket.
The theory behind this approach is that the values of different assets can move independently and often for different reasons.
Shares move in line with the fortunes and prospects of companies; bonds are most prominently influenced by interest rates; and property values, while also influenced by interest rates, are also more closely connected to the performance of the domestic economy.
Get the right asset allocation and you could make a healthy return, while also protecting yourself against the worst downturns in individual markets.
Find out more: How to invest - the Which? investment portfolios can help you decide on the right asset allocation for you
Step 2: Diversifying by sector
Say you held shares in a UK bank in 2006. Your investment may have been very rewarding, so you decided to buy more shares in other banks.
When the credit crunch hit the following year, sparking the banking crisis, the value of your shares in this sector (financials) would have tumbled.
So, once you've decided on the assets you want in your portfolio, you can diversify further by investing in different sectors, preferably those that aren’t highly correlated to each other.
For example, if the healthcare sector suffers a downturn, this will not necessarily have an impact on the precious metals sector. This helps to make sure your portfolio is protected from dips in certain industries.
Some investors will populate their portfolios with individual company shares directly but others will gain access to different sectors through managed funds like unit trusts and Oeics (Open ended investment companies).
Find out more: Different types of investment - the Which? guide to your investment options
Step 3: Spread your investments across the world
Investing in different regions and countries can reduce the impact of stock market movements. This means that you're not just affected by the economic conditions of one country and one government’s economic policies.
Different markets are not always highly correlated with each other - if the Japanese stock market performs poorly, it doesn’t necessarily mean that the UK’s market will be negatively affected.
However, you need to be aware that diversifying in different geographical regions can add extra risk to your investment.
Developed markets, such as the UK and US, aren't as volatile as those in emerging markets like Brazil, China, India and Russia. Investing abroad can help you diversify, but you need to be comfortable with the levels of risk involved.
Find out more: The Which? investment portfolios - see how weightings in different countries might look for different risk profiles
Step 4: Buy shares in lots of companies
Don’t just invest in one company. It might hit bad times or even go bust. Spread your investments across a range of different companies.
The same can be said for bonds and property. One of the best ways to do this is via a unit trust or Oeic fund.
They will invest in a basket of different shares, bonds, properties or currencies to spread risk around. In the case of equities, this might be 40 to 60 shares in one country, stock market or sector.
With a bond fund, you might be invested in 200 different bonds. This will be much more cost effective than recreating it on your own and will help diversify your portfolio.
Find out more: Use our dividend tax calculator to find out how much you'll pay in 2017-18.
Step 5: Beware of over-diversification
Holding too many assets might be more detrimental to your portfolio than good.
If you over-diversify, you might not end up losing much money, but you may be holding back your capacity for growth, as you’ll have such small proportions of your money in different investments to see much in the way of positive results.
It’s usually recommended that you hold no more than 30 investments (be it shares or bonds). If you're investing in funds, 15 to 20 should be a maximum.
Finally, for many investors - especially those without the time, confidence or knowledge to make their own investment decisions - professional financial advice is a must.
Find out more: Financial advice explained - the Which? guide