If you're experiencing a slow spring or temperatures have warmed but aren't quite to be trusted to remain that way, try this trick before planting heat-loving starter plants like tomatoes and peppers out in the garden.
Keep your pepper plants, tomato plants and other heat lovers planted in black plastic pots (or even black garbage bags). Set them out in the sun during the day, and bring them under cover at night. The black plastic pots will absorb the heat of the sun and warm the soil inside, making it much warmer than the ground soil and spurring your young plants to keep developing. Be sure the soil doesn't dry out; between the warmth and any wind you may need to water daily.
Once your weather and soil has warmed up, transplant the plants into the garden.