the only way to increase ur income in GC is to have a higher infra level in ur chosen income style be it commercial, agriculture, tax, mining or tax
also another way to get more income is to get more land from ur colonies.
you do this by exploring high land planets and making high land clusters from them.
there are no other ways to make more per turn income but u can also play the market but its very very risky and u will most of the time loose money instead of gaining it
also if u have tax or commecial as your income if you are out of damage protection you will recieve a 5% bonus added onto ur income whilst ur out of DP and still making a positive income
i hope this helps you with ur question.
if u need assistance with anything else please dont hessitate to post in teh HC again
regards and happy hunting
Aba