Use a packaged potting mix. If you see competing brands at the garden center--some sold by volume and some sold by weight, buy the one sold by volume. It's the best way to ensure you're getting a high quality, lightweight potting mix. You can fertilize using a slow-release fertilizer like Osmocote once or twice during the season, or a water-soluble fertilizer like Miracle Gro every other week. (Some potting mixes include a slow-release fertilizer, so additional fertilizer may not be necessary all season.) Pansies are cool-weather plants, and they tend to fizzle out when the summer heat comes along. They're usually planted in spring and fall for short-term color. Some old-fashioned petunias get ratty mid-season and can benefit from being cut back by half, but most of the petunias on the market these days just grow and bloom like gangbusters and you wouldn't dream of cutting them back. If in doubt, buy a different variety than you've grown before. The entire 'Wave' petunia collection is outstanding.