  We fit binary lens models to the data covering the initial part of real
microlensing events in an attempt to predict the time of the second caustic
crossing. We use approximations during the initial search through the parameter
space for light curves that roughly match the observed ones. Exact methods for
calculating the lens magnification of an extended source are used when we
refine our best initial models. Our calculations show that the reliable
prediction of the second crossing can only be made very late, when the light
curve has risen appreciably after the minimum between the two
caustic-crossings. The best observational strategy is therefore to sample as
frequently as possible once the light curve starts to rise after the minimum.
