How to deal with parasitic L's and C's?

Short Answer: Incorporate the parasitics into a filter structure.

You can't entirely eliminate the frequency rolloff due to parasitics, but you can reshape and push out the rolloff to a higher frequency. How is this possible? If there's a parasitic shunt C on the line, you can add series inductors or make the microstrip line inductive to form a "T" shaped low pass filter. Conversely, with a parasitic series L on the line, add shunt capacitors.