The better the piece, the greater the appreciation in value. Rather than buying lots of objects that aren't great, save up and buy a key piece of furniture or item. You need to learn as much as you can so that when you go to a market or shop you know more about the piece than the trader does. You will be rewarded by finding that special item at a really good price.

Know the details

Once you have knowledge of your subject, you'll know what details to look for. For example a highly collectable piece is the Charles and Ray Eames lounge chair and ottoman (all the Eames chairs are collectable but this one is especially so because it was produced as a luxury item). It was originally made in the States by Herman Miller but licensed copies were produced in Europe at the same time and they are in production again. The early Herman Miller chairs from the States have round black and silver stickers while the slightly later European stickers were oval. On the American chairs the gliders [feet] are silver and black, while the European ones are white and black. The arm rests on the early ones are around one inch thick, whereas the copies tend to be chunkier.

Go to auction

Auctions are a good place to buy because you are usually bidding against the trade buyers and so cutting out their mark-ups. There are no 20th century sales in Ireland, so you'll need to go to British auction houses such as Christies and Bonhams.

Research your subject

There are various ways of researching 20th century furnishings. Good websites on which you can shop and keep an eye on growing trends and prices are eBay.com and twentytwentyone.com. A great site for links to all the best Internet addresses is xxo.com. Some collectors' guide books are quite general and will give pictures rather than specific marks to look for. More detailed information is available in specialist books which you need to buy from specialist book websites. It's also worth buying books or design magazines from the period you are interested in. Talking to dealers is a great way of finding out about things. I've found this particularly in the area of 20th century design. It is still quite a specialist subject, so it's almost like a club and people are eager to share their knowledge.

The value of prints

Good pieces to collect are 20th century prints. High-quality prints by unknown artists are fairly inexpensive and will always have an inherent value of their own. However, prints by recognised artists, for example Vasarelly, although more expensive, are still affordable at around £500 for an original lithograph and are a better investment in the long run.

Two to watch

Furniture by George Nelson, a design advisor to Herman Miller, is increasing in value. Prices are rocketing but it's not too late to see a return. Danish architect and furniture designer Verner Panton is one of my favourites, and his pieces are still good value. He designed the red furniture in the white and red airport lounge featured in 2001: A Space Odyssey.

Original and untouched

Never change an original piece in any way. The exception is upholstery which should be researched carefully (don't change it unless it's really necessary, a certain shabbiness makes it look more authentic). You can still buy original fabrics for some furniture, for instance Florence Knoll made beautiful furniture in the 1950s and 1960s which is highly collectable. The company is still in business and produces its original fabrics. Florence Knoll furniture is very rare in Ireland and easier to find in the UK. There is a really good selection in the States, where there is a hard-core following of the designer's work. A Knoll two-seater will cost from $5,000.

Glass buys

Murano glass is becoming a collectable and is still available in Europe at marketstall level from around £10 up to hundreds of pounds. Always look for pieces that are signed and dated and with the original stickers. Holmegaard glass is also a collectible that is readily available in Europe, where it is not too expensive. Holmegaard is much more expensive in the States, so in the long term this should prove to be a good investment. The best way to get to know these pieces is to seek them out in places where they are labelled so that you will be able to recognise them at a later date in markets, and elsewhere, where the seller may not know what they have. The more you come into contact with original pieces the more you get a sense of the real thing.