Besides multipatterning, there's also immersion photolithography. By using a liquid with an index of refraction higher than air/vacuum, you can shrink the wavelength. With an index of refraction of 2, something with a vacuum wavelength of, say, 300nm becomes 150nm. That, combined with multipatterning, can help further reduce the feature size.

I think that immersion photolithography doesn't work too well with extreme UV because there aren't many liquids which are transparent in this region.

EDIT: to use water as an example, you can see here why 193nm is basically the limit for the technique of immersion photolithography. The absorption spectrum is extremely steep at 193nm, so at shorter wavelengths, essentially no light gets through: http://www1.lsbu.ac.uk/water/images/water_spectrum_2.gif