Greenhouse Cooling, Shade and Ventilation
Ventilation and shade are more important in summer. You could install electric fans but really you just need to be sure that your greenhouse has enough vents and door openings to allow adequate airflow. The general rule of thumb is 20% of the floor area should be reflected in ventilation, preferably spread throughout the greenhouse to promote the through flow of fresh air.
Auto Vent Openers allow the vents to be opened and closed according to the weather if you are not at home. Roof vents are common, and louvre vents in the lower walls also promote good airflow. Remember to leave enough space between your plants so they can breath, especially tomatoes.
As for shade, you might remember glasshouses being painted with a white water based mixture during summer months. This was done to diffuse the harsh sunlight and reflect some of the suns energy back out of the glasshouse, thus reducing the internal temperature and protecting your precious plants from being sun damaged and burned. An easier method of blocking the suns strong rays out of your greenhouse is by using a shade cover system. Winter Gardenz shade cover systems fit all types of greenhouse (even the old glass ones). Depending on the type of greenhouse you have you might not need to do this if you have a polycarbonate or plastic film greenhouse as these synthetic glazing materials are not as clear as glass and do a better job at diffusing the harsh sunlight.