I usually get stung on the side of my neck, as my curly hair acts like a net and bees just get caught in it. They also get in my helmet on road rides but the mesh in my LAS helmet used to stop them. Now I have a Specialized Prevail helmet with huge vents, and only my Heatsweats skullcap to protect me...
Never got one down the front of my shirt though. I get them down the back when I MTB. They go down as far as the where the top of the Camelback is and sting me there. Every summer without fail, two or three times.
I don't know how to stop them other than wearing a beekeeper suit. If the bugs get bad, a bandana made from very lightweight material could keep them out of your mouth if you don't mind looking like a reject from an old western.
Tzvia- rollin' slow...
Specialized Ruby Expert/mens Bontrager Inform RXL
Specialized SWorks Safire/mens Bontrager Inform RL
Giant Anthem-W XT-XTR/mens Bontrager Inform RXL
Fuji Newest 3 commuter/mens Bontrager Inform RL
Novara E.T.A commuter/mens Bontrager Inform RL