Deep slow breaths, but obviously not raising and lowering your rib cage as if you are in ante natal classes :P
Something i read or heard somewhere else was to play out the scene in your mind beforehand and imagine yourself there, talking confidently - taking control of situation. That's more psychological though and can be applied to relatinship situations, but was put to me as a way to overcome nerves when speaking publicly (e.g. speech in paacked room, all eyes on you)
If you're in some kind of social event (party) then flutter past and casually say something quick/snappy like hi how are you im good, quite thirsty off for a drink be right back. (obviously wait for his replies and dont say it all too fast) that way you're not comitted to a big convo with him and you can leave before you get tongue tied or if you feel nervous, without it becoming apparent. However, if a conversation blossoms from that then stick with it!