Juvenile Justice Improvement Act of 2008 - Amends the Juvenile Justice and Delinquency Prevention Act of 1974 to: (1) revise and add definitions under such Act relating to the treatment of juveniles held in custody while awaiting trial for criminal offenses; (2) require state plans under such Act to separate juveniles from the adult prison population, eliminate the use of dangerous practices used for holding juveniles in custody, and provide training of prison staff on techniques for effective behavior management of juvenile offenders; (3) provide incentive grants to states to adopt programs for the mental health treatment needs of juveniles in custody and for the placement of such juveniles in the least restrictive detention or correctional settings; and (4) prohibit the placement of juveniles who have not been charged with adult criminal offenses in secure detention or correctional facilities.