The strategy I would use is the following:
Goal is to get $100$ as sum. Among the digits $123456789$, pick and choose the sum close to $100$, such as $89$. Therefore I would attempt to get a value of $11$ from $1234567$ using different combinations.
When you start working on a smaller sum now (sort of like divide and conquer), you may get the desired result. (Of course there is no specific algorithm).
In order to get $11$, I have
$(1\times 23)-4+5-6-7 = 11$
$(1-2+3-4+5)\times 6 -7= 11$
$123-45-67 = 11$
Therefore
$(1\times 23)-4+5-6-7+89 = 100$
$(1-2+3-4+5)\times 6 - 7+89=100$
$123-45-67+89=100$
${\bf{Adding}}$ ${\bf{more}}$ to it: If we look at $78+9 = 87$ and instead of $89$, we seek the remaining $13$ to be derived from $123456$, and one way to get that is
$6+5+4-3+2-1=13$
Therefore
$78+9+6+5+4-3+2-1=100$