  We study classical two-dimensional frustrated Heisenberg models with
generically incommensurate groundstates. A new theory for the spin-nematic
"order by disorder" transition is developed based on the self-consistent
determination of the effective exchange coupling bonds. In our approach,
fluctuations of the constraint field imposing conservation of the local
magnetic moment drive nematicity at low temperatures. The critical temperature
is found to be highly sensitive to the peak helimagnetic wavevector, and
vanishes continuously when approaching rotation symmetric Lifshitz points.
Transitions between symmetry distinct nematic orders may occur by tuning the
exchange parameters, leading to lines of bicritical points.
