Cool summers here in the Northwest are great for spinach, lettuce and peas but tricky for tomatoes, eggplant, peppers and other warmth-loving vegetables. Cloches and mini-greenhouse frames help with warm air but for a good start make sure the soil is as warm as possible.
I've been covering my tomato and cucumber area with clear plastic for several weeks, a better choice than black plastic for warmth rather than weed-blocking. The sun's rays can penetrate down to the soil and the plastic keeps the warmth in at night.
Next step is to irrigate the area with water from a hose that's been left out in the sun, even on cool days. Then plant, cover with cloches and continue watering with warmed water.
And when the ground is warmed up, mulch with a layer of rocks or concrete blocks to radiate warmth at night and on cool days. Mulching with rocks isn't recommended in areas with warm summers because they get too hot, so it's a good deal here.