1. Start with fresh (or properly stored) seeds from reputable companies
Moisture and temperature fluctuations dramatically effect seed viability making it critical to use seeds that are either fresh from the grower or that have been stored properly.
A good rule of thumb is to store seed so that the temperature in degrees Fahrenheit + percent relative humidity < 100. Here in Colorado, the yearly averages for relative humidity swing between 35% and 65%. Thus, seed storage must be between 35 and 65 degrees.
According to Rodale Organic Life, the refrigerator is generally the best place to store seeds. To keep seeds dry, they suggest wrapping 2 heaping tablespoons of powdered milk in 4 layers of facial tissue, then putting the milk packet inside the storage container with the seed packets. Or adding a packet of silica gel. Replace every 6 months. When you’re ready to plant, remove seed containers from the refrigerator and keep them closed until the seeds warm to room temperature. Otherwise, moisture in the air will condense on the seeds, causing them to clump together.
When buying seed, check the date on the packet. The fresher, the better.
2. Use artificial lighting
Low light levels, like those inside most homes, cause weak and "leggy" plants. Since strong plants produce a strong harvest, give your baby plants the best head start possible with artificial lighting. My preferred light source for plant-starting is full-spectrum fluorescent bulbs. Although LED grow lights are becoming popular, I have not seen sufficient evidence (personal or research) to suggest that plant growth under LED is significantly better than fluorescent to warrant the higher upfront cost of LED, especially for short-term indoor seed-starting. (If you have had any seed-starting experience using LED, I'd love to hear your story in the comments!)
You can find most of the necessary supplies at Home Depot or Amazon. In previous years, I used a cheap, rectangular, 4-foot fluorescent light fixture hung from chain in my kitchen. The chain was attached to hooks in the ceiling, and I could lower or raise the fixture as needed as the seedlings grew. This is great if you plan to start lots of seedlings indoors and you have the space. Adjustable set-ups are important; light bulbs should be as close as possible to the seedlings without touching them. This is why incandescent bulbs are bad for seed-starting; heat-emitting bulbs will burn leaves and dry out soil.
For best results, use a timer, and set lights for 16 hours per day.
This year, I purchased a two-foot, T5 fluorescent grow system by Lithonia for just under $50. (I'll let you know how it goes!)
3. Use sterile soil in sterile containers
It might sound wasteful, but I usually end up purchasing new seed starting trays (cell packs) every year. To me, the risk of soilborne disease isn't worth the few bucks of savings from reusing trays. (At the end of the seed starting season, trays can be recycled.) Resist the catchy marketing of "alternative seed pots" made from fiber or the "Jiffy pellets". They're expensive and the natural fibers wick moisture away from the soil, drying out the seedlings. Natural materials also encourage mold growth. Although I have had relatively good success with making "pots" out of newspaper (talk about frugal), it doesn't compare to the reliability and cleanliness of lightweight seed cell trays. There are good reasons that your favorite local nursery uses them. If you are still adamant about reusing plastic pots, make sure to sterilize them before use in hot water and diluted bleach. Similarly, using a high-quality, sterile seed mix is just as important.
4. Provide air movement
In addition to sterile soil and trays, the best way to keep seedlings healthy and strong is providing air movement. Moist surface soil combined with stagnant, indoor air breeds mold that can stunt or even kill young seedlings. A small oscillating fan set on low is all you need, sweeping air across seedlings. Air movement also creates sturdy stem structures.
5. Get a heating mat
Germination rates are dependent on soil temperature. Onions and parsley, for example, germinate best at 75 degrees soil temperature. Tomatoes, peppers, and eggplant germinate best at soil temps of 85 degrees. Unless you plan on cranking up your home thermostat and wearing your bathing suit to breakfast, boost your germination rates with a seedling heating mat placed underneath your seed starting trays, and bookmark this handy-dandy seed starting temperature chart.