I agree with RChung---and not just because he is an engineer and a math genius who has spent a lifetime studying this stuff.
Your position on the bike is probably the biggest thing you can change, and the least expensive---all it takes is lots of suffering. A helmet is another relatively cheap option. However, if you get a really long-tailed TT helmet, every time you drop your head out of fatigue, that "aero" tail becomes an air-brake.
There are a couple different sites I have seen cited here which have lists of what "aero" gear you can get for how much cash and how much benefit you get in reduced drag .... none of it as far as I recall matters as much as staying tucked real low with your arms in and your elbows bent and tucked directly behind your hands with your forearms parallel to the earth so that you present the smallest frontal area.
All of this is possibly entirely wrong.