Express $1.\overline{234}$ as an improper fraction.
The trick to convert repeating decimals to fractions always involves recognizing the pattern of the repeating decimal and using it to your advantage. In this case, we can recognize that multiplying $1.\overline{234}$ by $1000$ gives $1234.\overline{234}$, a decimal with the exact same repeating portion. Thus, \[
(1000-1) \cdot 1.\overline{234} = 1000 \cdot 1.\overline{234} - 1.\overline{234} = 1234.\overline{234} - 1.\overline{234}
\]\[
\Rightarrow 999 \cdot 1.\overline{234} = 1233
\]\[
\Rightarrow 1.\overline{234} = \frac{1233}{999} = \frac{137 \cdot 9}{111 \cdot 9} = \boxed{\frac{137}{111}}.
\]