Simply writing out the numbers $1 - 9$ along the top of your paper can be a great help.
$5 \: 2$
$1 \; 9$
$5 \; 1$
$2 \; 9$
If the total has to be $100$, think about how the four digits you choose could add to make the $0$ in the units column, then look at the tens column.