IPhone 3GS running slow not working well?

Well I make a call and at the end I slide end call and it just freezes in that position for like 10 seconds. Also I press play or pause on the iPod and it doesn't registra until after 5 or so seconds. There r also acouple of other similar problems and delays so Wat do u think is the best thing to do to make it fast like it was. ( I bought it with a plan from virgin) thx :)