Sometimes i get a little sick of playing GT3, but not to the point where i just totally disregard it, and permenatly stop playing. If this happens i normally go play a stratergy game on PC like StarCraft or WarCraft and when i get sick of those games i go back to GT3.
Even if i dont feel like playing, i normally force my self to play so i dont loose my skill and most of the time i get back into it and feel like playing again.
PLUS, i have paced myself playing the game, meaning that i didnt just get the game, and buy all the best cars at once. My freind on the other hand got the game before me and just went nuts, buying all his favourite cars. (He got sick of it raelly quick) Even if i have the credit to buy a car that i really want, i buy another car. This way i take advantage and get the most out of every car that i buy.
Hope this helps.