what is the best way of sampling an exponential decay?
If you know that this is the only signal you're observing, than literally any two sampling points from $[t_\text{min};t_\text{max}]$ will do.

This is a problem involving a bijective function with two unknowns (assuming $C\in \mathbb R$). So, two observation will, as always, do. 

To minimize the error, you'd choose sampling times that are as far apart as possible, considering that your function is strictly monotonous.