(FORTUNE Magazine) – Advertisers have turned the heat up higher than ever before on Madison Avenue. With television ads becoming ever shorter and more crammed, they're demanding that their ads stand out from everybody else's. To keep their clients happy, ad agencies around the country loaded up on cash and went on a shopping spree, raiding competitors' creative stars. Says Roger P. Mosconi, 45, who conceived the Michael J. Fox ads for Diet Pepsi at BBDO before switching in October to Lintas: USA to handle Diet Coke International: ''All of a sudden I got 11 job offers because the clients say, 'This guy does advertising people look at.' '' He won't say how much he took to switch, but sources estimate that star writers and art directors are offered raises of up to 40% to jump ship. Some of them make $500,000 a year.