If your pants have a removable belt, just thread the belt through the suspender holes at the appropriate places. If the belt loops are in the right spot, put the two suspender buttonholes on either side of the belt loop so the suspenders don't slide along the belt too far.
Code:
| |
| | <-- Suspenders
// \\
// \\
===O==|==O==== <-- Belt, suspender buttonholes, and a belt loop in the middle
Not as elegant as Keeps' solution, since his way guarantees the suspenders will stay anchored to wherever you sew them in. That said, I've never had the suspenders move along the belt enough to be a problem.