  We report on a scheme to improve the pointing stability of the first order
beam diffracted by an acousto-optic modulator (AOM). Due to thermal effects
inside the crystal, the angular position of the beam can change by as much as 1
mrad when the radio-frequency power in the AOM is reduced to decrease the first
order beam intensity. This is done for example to perform forced evaporative
cooling in ultracold atom experiments using far-off-resonant optical traps. We
solve this problem by driving the AOM with two radio-frequencies $f_1$ and
$f_2$. The power of $f_2$ is adjusted relative to the power of $f_1$ to keep
the total power constant. Using this, the beam displacement is decreased by a
factor of twenty. The method is simple to implement in existing experimental
setups, without any modification of the optics.
