2007 toyota avalon lxs with 42,000 miles gas mileage?

[bigboi64]

can somebody please help me with my gas mileage question,i have noticed that in my 2007 toyota avalon sometimes it gives me 24 miles per gallon on the highway and some times it has given me 31 miles per gallon on the highway id there something i could do to make shore it always gives me the most highway miles per gallon