Here's a quick and easy solution (albeit an ugly one...at least in the following example.)
Put an absorbent wicking material into the soil and extend it into the water (I used yard. Hot pink works best *kidding*) Then add moist soil to the top half of the bottle and water in the bottom.
The soil will drain into the bottom if it is too damp and will wick up water from the bottom if is to dry.
The soil will stay wet enough on it's own. When you need to add water to the system, just lift up the top portion and add water to the bottom. Easy.
Mine are "dirty" because I had way too much water in the soil at the onset and as the water drained through the soil into the water below. No biggie. Just a little ugly.
You can also use bigger bottles and/or cut them better than I did. Mine look horrible, but these are my first attempt. I was practicing with used water bottles but will save bigger bottles (preferably ones that are large enough that more of the top can fit into the water) for my main seed starts this spring.
I am growing cilantro in these containers because cilantro is a great test of the system. It takes forever to germinate (more than a week!) so it's the ultimate test of keeping soil moist.
I am happy to say that the cilantro finally germinated and the soil stayed damp. I only had to refill the bottom of the bottles once per week.
My windows provide almost no UV light, so I am anxious to see if they will survive and be able to transplant into larger pots. Either way, the system seems to work as it should and I will be using these for some of my larger seedlings this spring.