Get some colour into your greenhouse by adding some flowers These in turn will attract the ladybirds. Some are better than others, therefore I wont go into detail about which ones to get/grow... someone else can do that ...
http://www.laurasorganics.co.uk/articles.php?article=4
Personally I have Viper Bugloss and Nasturtiums at the entrance of my poly tunnel and greenhouse. Having just done my tomatoes this morning, I can assure you that these encourage the ladybirds in as they were covered in them. Make sure you leave the doors open as you don't want it too hot.