Credit card sign up bonuses are the fastest way to accure frequent flyer miles. The sign up bonuses allow for massive earning of points you can transfer to miles. Make sure to look for targeted offers. These offers give a smaller spend for a larger bonus.
If you like hotels stay with one chain if possible, customer loyality programs are great ways to get free nights.
Make sure you are signed up for all active promotions ran by frequent flying programs. An example would be Delta and AirBnb's current partnership. Delta and AirBnB
AirBnb Signup use my link to sign up if not a member of Airbnb. You get a $35 credit, and I get one when you stay and use it.