Two major challenges to effective transition planning are 1) failure to start early enough and 2) the fact that the adult service system in every state is different and there is no state in the union for which there is an actual, legislative entitlement for adult services (although California does come close). So the first recommendation is start the process early -- ideally by the time your child is 14 years of age.
To start, parents must educate themselves about what services are available in their state, how best to access these services and what funding streams help support which services (as that may dictate eligibility). The best source of information on this topic will be parents of adult children with Autism Spectrum Disorders (ASDs) in your area. These groups can often be accessed through the local chapter of the Autism Society of America (ASA).
If you are unhappy with your district’s resources and response, your recourse lies in the power of the Individualized Education Program (IEP) process. Take advantage of that legal document to develop and implement a comprehensive, individual transition plan for your child. It may not be easy (and in many cases won't be), but nothing is legally comparable to the IEP after a child turns 21. So please, use it before you lose it.