Sourdough is basically fermented flour and water, fed with more flour and water on a regular schedule. I came cross this video which explains the process.

At this time of year, make sure your water is warm, around 80-85 F (26-29 C), and it will kickstart the process.

Another trick: rise the starter in a microwave oven or other enclosed space and place a cup of just-boiled water inside. Don't turn on the microwave: you just want to create a "proofing box" (enclosed space) that keeps the starter warm. 

You can also use whole wheat flour in place of the rye, or all rye or all whole wheat, or spike the starter with a tablespoon of honey. I've tried any number of methods. But whole grain flours tend to be much more active (which is why they're used), so keep an eye on the stuff every 12 hours or so. Another general rule: the warmer it is, the faster it ferments, unless it's so hot that the natural yeast and bacteria are killed. If the water feels warm to the touch, it should be fine.