Why does the plaster crack beneath the window sill?
I've seen this happen often at the corners of windows and doors.  My first question would be - is there a seam in the plasterboard where the crack is occurring?  If not, is the crack just through the skim coat or is the board underneath actually cracked?

If you just plaster over the crack, it will certainly crack again.  I would try either taping over the crack with mesh tape and plastering over the tape or if it's just a hairline crack, caulk it and paint it.  The caulk should be flexible enough not to crack again.