The easiest way and one which causes the least loss of yield is to give them a 24 hour dark period.
I understand totally what Jondamon is suggesting but there have been studies done which suggest that by giving 13/14 hours of either day or night for an extended period (more than 1 or 2 days) can cause a 20% loss of yield.
The 24 hour dark period, especially if later in flower funnily enough can actually increase yield since the plant thinks it is coming to the end of its life sooner than it expected so it can increase resin production. This is the way I have done it when ambient temps have got too hot for lights on in the daytime and I have never lost yield or had one go hermie on me for this reason.
As with most things to do with growing, you will hear differing opinions, most of which sound plausable. Your "job" as a grower is in deciding which you believe is most beneficial to your plants.
Green mojo dude, W