Be sure to add LIGHT!
Unless you have a good source of southern light, you may want to use fluorescent lights to help your seedlings grow. In the wintertime, many seedlings are not happy near windows; the drafts can be deadly (the Bio Dome and other domed trays help avoid this problem). If possible, use grow lights such as our economical Folding Light Set. Place the young seedlings under lights as soon as they straighten up after unfurling their first set of leaves. You can put them much closer to the source of light than you'd expect -- just a few inches is ideal for many vegetables and flowers. (Of course, as they grow, keep an eye on the young plants and adjust the light upward so the tender leaves don't touch or get TOO close to the light!)
You will find yourself using your grow lights more than you imagined. They're great for pepping up stressed-out houseplants, especially those that need lots of natural light and just don't get their fair share in winter. And all your seedlings will grow more vigorously, with greener foliage and a more compact size (less stem between the leaves!), under plant lights.