Compute context free grammars for twice the amount
One general approach would be to try writing some initial strings and try to figure out a pattern. In my minimal experience, a lot of the times the strings I miss are of smaller lengths.

The initial strings for this would be:

$(\epsilon, aab, aaaabb, aaaaaabbb, ...)$ and so on.

Here we notice that: a) The string always has to start with an $a$ and b) Every time we generate an $b$, there should be two $a$'s corresponding to it.  

So, we can write it as:

$$S \rightarrow \epsilon \  | \ aaSb$$