  Weak lensing surveys provide a powerful probe of dark energy through the
measurement of the mass distribution of the local Universe. A number of
ground-based and space-based surveys are being planned for this purpose. Here,
we study the optimal strategy for these future surveys using the joint
constraints on the equation of state parameter wn and its evolution wa as a
figure of merit by considering power spectrum tomography. For this purpose, we
first consider an `ideal' survey which is both wide and deep and exempt from
systematics. We find that such a survey has great potential for dark energy
studies, reaching one sigma precisions of 1% and 10% on the two parameters
respectively. We then study the relative impact of various limitations by
degrading this ideal survey. In particular, we consider the effect of sky
coverage, survey depth, shape measurements systematics, photometric redshifts
systematics and uncertainties in the non-linear power spectrum predictions. We
find that, for a given observing time, it is always advantageous to choose a
wide rather than a deep survey geometry. We also find that the dark energy
constraints from power spectrum tomography are robust to photometric redshift
errors and catastrophic failures, if a spectroscopic calibration sample of
10^4-10^5 galaxies is available. The impact of these systematics is small
compared to the limitations that come from potential uncertainties in the power
spectrum, due to shear measurement and theoretical errors. To help the planning
of future surveys, we summarize our results with comprehensive scaling
relations which avoid the need for full Fisher matrix calculations.
