You blush because you feel uncomfortable about something that is happening, then you feel more uncomfortable because you think people know you are uncomfortable because of how much you are blushing.. A bad cycle!
There are no tricks that will make you feel more comfortable, imagining people naked for instance, because you are only tricking yourself, and you know it's a trick and frankly if everyone WAS naked you'd probably blush more! And tricks only serve to take you away from the present moment, where really you should be embracing the present moment, only by embracing it will you to enjoy it. Trying to forget it is not dealing with reality.
The only way to properly deal with situations like this to to call out what's real - your red face - tell everyone exactly how you blush when put on the spot, and how much you dislike speaking in a group, make a joke about the guy next to you having to take his jumper off because your cheeks are so hot.
Once you have spoken about it then you really have nothing to be embarrassed about, it's all out there. Be the blusher of the group and try to embrace it, it's and you should try to love yourself for this trait - that's the only 'cure'.