Moving your saddle slightly forward (which may require an offset seatpost) helps you extend comfortably onto the aero bars.
Buying aero wheels is the easiest way to reduce drag. But the seconds saved will cost you: Zipp's 808/404 pairing (zipp.com) runs $2,210.
Is the down tube the most aerodynamic spot for a water bottle? No. But leaving it here makes drinking easier, which is more important.
Shopping for a do-it-all bike? Go with the Cervélo Soloist Carbon ($5,000; cervelo.com), which has road-bike geometry and aero tubes.
Clip-on bars like Zipp's Vuka Clip ($150) get you aero with minimal mechanic time.
More surface area on aero wheels makes you vulnerable to crosswinds. Be conservative up front—the Zipp 404 rim is a reasonable 55 mm deep.