Put them next to a window that gets plenty of light. It's not enough just to put your plant next to a window. You need to make sure it has access to plenty of light. Make sure it's not a window that has a huge tree right outside blocking light. Use artificial light.
Fluorescent lighting is the best light source.
You want to provide a decent amount of light without the heat, and fluorescent bulbs do just that. If you have a small or average sized plant, a standard fluorescent bulb will do just fine. But if you have larger plants, you want to get bulbs that produce as much light as possible. Put your plants less than a foot from the artificial light. You want it to get plenty of light.