Create a composition with a low frame rate. Nest the composition containing your footage layers in this lower-frame-rate composition.
Or, you can use the Posterize Time effect, which is a bit simpler but a lower-performance method.
Details here:
http://helpx.adobe.com/after-effects/using/time-effects.html#posterize_time...
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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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