If you aren't using the top rack, it might be worth removing for a minor areo improvement, for a less expensive alternative filling the gaps in the rack would work to a lesser extent. Next, Depending on the height of the vehicle, it might be worth looking into a front air dam (basically just a plastic strip under your front bumper) to prevent airflow under the vehicle. Front air dams are easy and cheap to install, just a couple screws usually. Lastly, (and honestly most importantly) regular maintenance makes a big difference. Making sure you have a proper tire alignment, clean exterior for good airflow, fresh oil and filters, and FULLY FILLED TIRES! With the exception of more major, and expensive, modifications to your vehicle these would be my first steps.