For someone who once predicted that everybody would be famous for 15 minutes, Andy Warhol has had far more than his fair share. And there are no signs of his popularity declining.
"He was one of the great artists of the 20th century," says Susan Harris, director of the modern and contemporary prints department at auction house Sotheby's. "His imagery has such appeal. Young people can relate to it and it's instantly recognisable. Anyone who bought one of his prints in 1994 will certainly have seen a return on their investment."
Murray Macaulay of rival auctioneer Christie's agrees: "We've seen a definite increase in the value of Warhol's work, particularly in the last two or three years. There was an exhibition at the Tate Modern three years ago which helped boost interest, and it's looking very strong at the moment."
Tom Sokolowski, director of the Andy Warhol Museum in Pittsburgh, says part of Warhol's genius, and a reason for his enduring popularity, was that he produced accessible art.
"He used a style people were comfortable with," he says, "with subjects that people were familiar with, and in different sizes so that at least one would fit in their homes. He once said: 'Art galleries should be like department stores', letting you go in and shop for what you wanted. He understood the market and his popularity has lasted."
Warhol started off working in advertising in New York. Within three years he was the highest- paid illustrator in the city. A sale this week at Sotheby's includes a number of works that show his ability as an illustrator, along with his groundbreaking printmaking. A series based on the hammer and sickle (estimated value: £8,000 to £10,000) is one example, next to the unusual Details of Renaissance Paintings (£2,500 to £4,000).
One advantage of buying Warhol prints, as opposed to his paintings, is affordability. "He was one of the first artists to use silk-screen printing seriously," says Ms Harris. These are not just mass-produced prints. Each one is signed by him and stamped with a number. With the Mao set and the Marilyn set, for example, he did 10 different colour combinations and prin-ted 250 of each. It means that you can buy a genuine portrait by a great artist for under £10,000."
The Sotheby's sale includes sets of icons such as Marilyn Monroe (estimated value £230,000 to £260,000 for the whole set of 10), Chairman Mao (£2,000 to £8,000 each) and Liz Taylor (£4,000 to £6,000 each). It also includes what Warhol called his "10 Jews": photographs of personalities such as Einstein, Freud and the Marx Brothers (£30,000 for the complete set).
Mr Sokolowski says the artist's stock will continue to rise. "Warhols have such a personal style that, even if you hate them, you'd know a Warhol as soon as you saw one," he says. "If you saw one on someone's wall, you'd know they had money. It's a label - and Andy understood labels better than anyone."
Mr Macaulay agrees: "When you see the portraits of Marilyn Monroe, you think of Warhol before you think Monroe. He is so associated with the mid-20th century that the popularity of his work will certainly last."
The original paintings are the most valuable. One of Mick Jagger sold recently for $150,000, while one of Monroe fetched $17.5m. His first Campbell's Soup series, which originally went on the market in 1961-62 for $200, didn't sell at the time. Later, they went for for $37m.
Even Warhol-related items could be worth investing in. Christie's has a poster from an exhibition at the Tate in 1971, signed by the artist, for sale this week: it is expected to fetch £500 to £700. A couple of designs for the jacket of his book The Philosophy of Andy Warhol, which he also signed, have an estimated value of £1,500 to £2,000.
As with most collections, aim for the best-quality prints when investing in Warhol's works. "The condition of the prints that come up for sale varies a lot," says Ms Harris. "But if you get one in good condition, you'll know you have an original graphic work by an artist who was a master printer."
From £500 for a Warhol-related item to over £20m for an original painting of an icon.
30 June: Pop Art 1954-1974, Christie's, London. 1 July: Andy Warhol and the Pop Generation, Sotheby's, London (viewing starts today).