How can I maximize the amount of energy an antenna is receiving?
By the principle of reciprocity, antennas work the same way in receive and transmit directions. So if you make a directional antenna that's better at transmitting in a particular direction, it's better at receiving in that direction by the same amount.

If your goal is maximizing power received, then a useful concept is effective aperture. This is the size of the metaphorical "net" the antenna uses to capture energy. A bigger net means capturing more energy. If you are familiar at all with the term "aperture" in optics and photography, it is a similar concept.

A somewhat counter-intuitive mathematical fact is that effective aperture (\$A_{eff}\$) and gain (\$G\$) are related:

$$ G = \frac {4 \pi A_{eff} } { \lambda^2 } $$

\$\lambda\$ is the wavelength, and people tend to develop a misconception here: that the physics of energy transmission are somehow different depending on frequency. They aren't. All electromagnetic radiation decreases with distance according to the inverse square law, be it AM broadcasts, visible light, or gamma radiation. See Is free space path loss dependent on frequency? and Why is antenna aperture a function of wavelength?

What this equation is telling you is this: as frequency decreases and gain remains constant, the aperture increases. But a half-wavelength dipole for 750 kHz is also physically much larger than a half-wavelength dipole for 2.4 GHz, so although they each have the same gain, it would make sense that the 750 kHz dipole has a larger aperture.

Another counter-intuitive result is that an ideal dipole which is infinitesimally small has about the same gain (and effective aperture) as a half-wavelength dipole: 1.76 dBi compared to 2.15 dBi, respectively.

If that's true, then why don't we use infinitesimally small dipoles everywhere? We could save a lot of space. The reason is that to efficiently couple energy with this tiny antenna, you need some kind of matching network, and those parts introduce losses, degrading the efficiency of the system.

But this is still insightful: if you want to maximize received power, focus on minimizing losses first. With the tiny powers you will capture from an antenna, you probably need it to be as efficient as possible, so design for simplicity since each component adds loss. And match your load to the antenna impedance to make the transfer of energy as efficient as possible.